Applied Science, Faculty of
Chemical and Biological Engineering, Department of
DSpace
UBCV
Bi, Wei
Doctor of Philosophy - PhD
University of British Columbia
Ammonia (NH₃) is critical to sustaining agriculture for the rapid growth of global population. This thesis focuses on NH₃ electrosynthesis via the electrochemical nitrogen reduction reaction (ENRR) and develops customized flow electrolyzers for aqueous and non-aqueous ENRR conditions. The ENRR performance with commercial or research electrocatalysts was firstly evaluated in aqueous media, but the prevalence of false positives from contaminated cell components masked the genuine ENRR assessment. Contributing to the observed false positives, NH₃ or ammonium (NH₄⁺) crossover through the separator was then investigated. Despite frequent appearances in the literature, Nafion as a cation-exchange membrane is incapable of completely separating NH₃/NH₄⁺ and acts as a possible source or sink of contamination. The behaviors of NH₃/NH₄⁺ that include crossover, volatilization, and adsorption in Nafion were systematically evaluated under a variety of electrolyte compositions, operation conditions, concentration gradients, and feed locations. Additionally, an anion-exchange membrane and microporous polypropylene separators were evaluated as alternatives to the Nafion separator. However, only the hydrophobic Celgard separator demonstrates satisfactory NH₃/NH₄⁺ separation at open-circuit conditions. Lastly, a three-compartment cell was used to evaluate the non-aqueous lithium-mediated ENRR performance of gas-diffusion electrodes with a carbon-based reaction layer: carbon-supported Pt, carbon black, and graphite. The ENRR results achieved in tetrahydrofuran-based electrolyte can generally achieve one-order of magnitude higher NH₄⁺ concentration than in aqueous media, thus avoiding false positives. The current density (−5 mA cm⁻²) and the EtOH concentration (0.5 vol%) were optimized with a current-cycling operation strategy using the C45 carbon black cathode. Replacing the Nafion binder with polyvinylidene difluoride further improves the LNRR performance to 3.11 ± 0.41 μmol h⁻¹ cm⁻² in NH₃ production rate and 5.0 ± 0.65% in Faradaic efficiency that are comparable to or exceeding previous literature reported results. Since Li plating causes exfoliation of the electrode, poor LNRR durability was observed at −5 mA cm⁻². However, reducing the current density to −3 mA cm⁻² achieves a stable operation for over 8 h owing to the balanced Li plating and dissolution rates that mitigates the reaction layer delamination.
IMPACTS OF ELECTROLYZER DESIGN, OPERATION, AND MEASUREMENT ON AMMONIA ELECTROSYNTHESIS IN AQUEOUS AND NON-AQUEOUS MEDIA by Wei Bi B.A., The University of British Columbia, 2019 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (CHEMICAL AND BIOLOGICAL ENGINEERING) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) March 2024 © Wei Bi, 2024 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled: Impacts of Electrolyzer Design, Operation, and Measurement on Ammonia Electrosynthesis in Aqueous and Non-aqueous Media submitted by Wei Bi in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemical and Biological Engineering Examining Committee: David P. Wilkinson, Professor, Chemical and Biological Engineering, UBC Co-supervisor Elöd Gyenge, Professor, Chemical and Biological Engineering, UBC Co-supervisor Vikramaditya G. Yadav, Associate Professor, Chemical and Biological Engineering, UBC Supervisory Committee Member Chester (David) Upham, Assistant Professor, Chemical & Biological Engineering, UBC University Examiner Jongho Lee, Associate Professor, Civil Engineering, UBC University Examiner Additional Supervisory Committee Members: Edouard Asselin, Professor, Materials Engineering, UBC Supervisory Committee Member iii Abstract Ammonia (NH3) is critical to sustaining agriculture for the rapid growth of global population. This thesis focuses on NH3 electrosynthesis via the electrochemical nitrogen reduction reaction (ENRR) and develops customized flow electrolyzers for aqueous and non-aqueous ENRR conditions. The ENRR performance with commercial or research electrocatalysts was firstly evaluated in aqueous media, but the prevalence of false positives from contaminated cell components masked the genuine ENRR assessment. Contributing to the observed false positives, NH3 or ammonium (NH4+) crossover through the separator was then investigated. Despite frequent appearances in the literature, Nafion as a cation-exchange membrane is incapable of completely separating NH3/NH4+ and acts as a possible source or sink of contamination. The behaviors of NH3/NH4+ that include crossover, volatilization, and adsorption in Nafion were systematically evaluated under a variety of electrolyte compositions, operation conditions, concentration gradients, and feed locations. Additionally, an anion-exchange membrane and microporous polypropylene separators were evaluated as alternatives to the Nafion separator. However, only the hydrophobic Celgard separator demonstrates satisfactory NH3/NH4+ separation at open-circuit conditions. Lastly, a three-compartment cell was used to evaluate the non-aqueous lithium-mediated ENRR performance of gas-diffusion electrodes with a carbon-based reaction layer: carbon-supported Pt, carbon black, and graphite. The ENRR results achieved in tetrahydrofuran-based electrolyte can generally achieve one-order of magnitude higher NH4+ concentration than in aqueous media, thus avoiding false positives. The current density (−5 mA cm−2) and the EtOH concentration (0.5 vol%) were optimized with a current-cycling operation strategy using the C45 carbon black cathode. Replacing the Nafion binder with polyvinylidene difluoride further improves the LNRR iv performance to 3.11 ± 0.41 μmol h−1 cm−2 in NH3 production rate and 5.0 ± 0.65% in Faradaic efficiency that are comparable to or exceeding previous literature reported results. Since Li plating causes exfoliation of the electrode, poor LNRR durability was observed at −5 mA cm−2. However, reducing the current density to −3 mA cm−2 achieves a stable operation for over 8 h owing to the balanced Li plating and dissolution rates that mitigates the reaction layer delamination. v Lay Summary Ammonia (NH3) has been used globally for artificial fertilizers, but its main production method, the Haber-Bosch process, is a significant source of CO2 emission. Therefore, a more sustainable manufacturing process is urgently needed, which could be achieved by electrochemically reducing N2 directly to NH3. This research develops electrolyzers suitable for NH3 electrosynthesis in aqueous and non-aqueous media. Despite multiple accounts in the literature, nitrogenous contaminants confuse the detection of genuine products from N2 under aqueous conditions. The commonly used separator, Nafion, cannot prevent NH3 or ammonium ion (NH4+) crossover or adsorption, which increases the possibility of false positives especially at low NH3 concentrations. Alternatively, non-aqueous NH3 electrosynthesis was proven to be more reliable than the aqueous case by using a gas-diffusing electrode prepared with a low-cost carbon reaction layer. Modifying the carbon material, binder, electrolyte, and electrolysis conditions shows clear improvements in the NH3 electrosynthesis performance and durability. vi Preface The work included in this thesis, including the literature review, experimental design and execution, data analysis and visualization, results discussion and interpretation, and thesis writing, was completed by Wei Bi under the co-supervision of Professor David Wilkinson and Professor Elöd Gyenge at the Department of Chemical and Biological Engineering at the University of British Columbia. A version of Chapter 1 has been published as: • W. Bi, N. Shaigan, A. Malek, K. Fatih, E. Gyenge and D. P. Wilkinson, “Strategies in cell design and operation for the electrosynthesis of ammonia: status and prospects”, Energy Environ. Sci., 2022, 15, 2259–2287. I collected the literature results, performed the data analysis and summary, and wrote and modified manuscript. All co-authors reviewed and provided feedback on the manuscript. Part of Chapter 4 has been published as: • W. Bi, E. Gyenge, D.P. Wilkinson, “Crossover, volatilization, and adsorption of ammonium ions in a proton-exchange membrane electrolyzer in relation to electrochemical ammonia production”, Chem. Eng. J., 2023, 478, 147359. I performed the experiments, collected and analyzed data, and wrote and modified the manuscript. E.G. and D.P.W. provided funding, designed and conceptualized the experimental work, and reviewed the manuscript. Part of Chapter 4 is drafted as a manuscript and currently under review as: • W. Bi, W. Bao, E. Gyenge, D.P. Wilkinson, “Ammonium Crossover as a Function of Membrane Type and Operating Conditions in Flow Cells for Ammonia Synthesis and Water Treatment Applications”, under review. I performed the experiments, collected and analyzed vii data, and wrote and modified the manuscript. E.G. and D.P.W. provided funding, designed and conceptualized the experimental work, and reviewed the manuscript. A version of Chapter 5 is currently in preparation for submission as: • W. Bi, W. Bao, E. Gyenge, D.P. Wilkinson, “Lithium-Mediated Nitrogen Reduction in a Flow Electrolyzer Cell Using Gas Diffusion Cathode with Carbon-Based Reaction Layer”, drafted manuscript. I performed the experiments, collected and analyzed data, and wrote and modified the manuscript. E.G. and D.P.W. provided funding, designed and conceptualized the experimental work, and reviewed the manuscript. The work has also been presented at the following conferences: • W. Bi, E. Gyenge, D.P. Wilkinson, N. Shaigan, A. Malek and K. Fatih, “Determining the Influence of Catalyst Layer Architecture and Reactant Flow in an MEA for the Electrochemical Nitrogen Reduction Reaction Under Ambient Conditions”, Electrochemical Society Conference 241st Vancouver, 2022. I attended the conference and gave the presentation. All the co-authors reviewed the presentation and provided feedback. • W. Bi, E. Gyenge, D.P. Wilkinson, “Investigating Ammonium Ion Behavior in a Flow-cell Electrolyzer and Implications for Selecting Appropriate Separators for Ammonium Electrosynthesis”, Canadian Chemistry Conference and Exhibition Vancouver, 2023. I attended the conference and gave the presentation. All the co-authors reviewed the presentation and provided feedback. viii Table of Contents Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ....................................................................................................................... viii List of Tables ............................................................................................................................... xii List of Figures ............................................................................................................................. xiv List of Symbols ......................................................................................................................... xxiii List of Abbreviations .................................................................................................................xxv Acknowledgements ................................................................................................................. xxvii Dedication ............................................................................................................................... xxviii Chapter 1 Introduction..................................................................................................................1 1.1 Research Objectives and Thesis Layout ......................................................................... 1 1.2 General Overview of Ammonia Electrosynthesis ........................................................... 3 1.3 Basic Electrochemical Cell Layouts ............................................................................. 18 1.4 Cell Component Design ................................................................................................ 23 1.4.1 Separator ............................................................................................................... 23 1.4.2 Liquid Electrolyte ................................................................................................. 28 1.4.2.1 Solute Species in Liquid Electrolyte ................................................................. 28 1.4.2.2 Solvent Type and Property in Liquid Electrolyte ............................................. 33 1.4.2.3 Li-Mediated Non-Aqueous Systems ................................................................. 40 1.4.3 Cathode Design ..................................................................................................... 45 ix 1.4.3.1 Catalyst Layer Surface Morphology ................................................................. 45 1.4.3.2 Substrate for the Catalyst Layer Deposition. .................................................... 49 1.4.3.3 Binder or Coating Materials.............................................................................. 53 1.5 Cell Operating Conditions ............................................................................................ 55 1.5.1 Temperature .......................................................................................................... 55 1.5.2 Pressure ................................................................................................................. 57 1.5.3 Gas and Liquid Flow Conditions .......................................................................... 60 1.6 Introduction Summary and Outlook ............................................................................. 63 Chapter 2 Experimental ..............................................................................................................65 2.1 Flow Cell Electrolyzer for the ENRR in Aqueous Media ............................................ 65 2.1.1 Electrode preparation ............................................................................................ 65 2.1.2 Physical Characterization...................................................................................... 67 2.1.3 Electrolyzer Cell and Flow Configurations .......................................................... 71 2.1.4 Electrochemical measurements ............................................................................. 75 2.1.5 NH4+ Crossover Tests ........................................................................................... 76 2.1.6 MEA conditioning for NH4+ crossover in Na2SO4 ............................................... 80 2.2 Flow Cell Electrolyzer for the ENRR in Non-aqueous Media ..................................... 81 2.2.1 Electrode preparation and characterization ........................................................... 81 2.2.2 Electrolyte Preparation.......................................................................................... 83 2.2.3 Electrolyzer Setup and Operation ......................................................................... 84 2.2.4 LNRR Experiments with the Flow Cell Electrolyzer ........................................... 86 2.2.5 Half-cell Experiments in a Single-compartment Glass Cell ................................. 89 2.3 Analytical Methods ....................................................................................................... 92 x 2.3.1 Materials and Chemicals ....................................................................................... 92 2.3.2 NH3 Quantification in Aqueous Solutions ............................................................ 92 2.3.3 NH3 Quantification in Non-aqueous Solutions ..................................................... 94 2.3.4 Additional Analytical Methods ............................................................................. 98 2.4 Performance Metrics Calculation and Unit Conversion ............................................. 101 Chapter 3 Identification of False Positives in the Aqueous ENRR .......................................103 3.1 Introduction ................................................................................................................. 103 3.2 Results and Discussion ............................................................................................... 103 3.2.1 Au/KB GDE as the cathode ................................................................................ 103 3.2.2 MoS2 GDE as the cathode ................................................................................... 106 3.3 Conclusion .................................................................................................................. 111 Chapter 4 Interactions between NH3/NH4+ and Separator Materials in the Flow Cell Electrolyzer .................................................................................................................................112 4.1 Introduction ................................................................................................................. 112 4.2 1-ppm NH4+ Crossover with Nafion N112 in Different Electrolyte Compositions .... 118 4.2.1 NH4+ Crossover through Nafion N112 in H2SO4 ............................................... 118 4.2.2 NH4+ Volatilization through Nafion N112 in NaOH .......................................... 119 4.2.3 NH4+ Adsorption and Desorption in Nafion N112 in DI H2O ............................ 121 4.2.4 NH4+ Crossover though Nafion N112 in Na2SO4 ............................................... 125 4.2.5 Implications for NH4+ Electrosynthesis in Flow Electrolysis Cells .................... 129 4.3 NH4+ Crossover at Elevated Current Densities and Concentrations ........................... 132 4.3.1 NH4+ Crossover with N112 in Cathode-fed Operation ....................................... 132 4.3.2 pH Polarization and Correlation with NH4+ Transport ....................................... 133 xi 4.3.3 NH4+ Crossover with N112 in Anode- and Symmetric Fed Operation .............. 139 4.3.4 Recovery Efficiency of NH4+ .............................................................................. 143 4.4 Importance of MEA Conditioning for Consistent Results .......................................... 146 4.5 NH4+ Crossover with Alternative Separators .............................................................. 150 4.6 Conclusion .................................................................................................................. 156 Chapter 5 Lithium-Mediated Nitrogen Reduction in a Flow Electrolyzer Cell Using Gas Diffusion Cathode with Carbon-Based Reaction Layer .........................................................159 5.1 Introduction ................................................................................................................. 159 5.2 Improving the LNRR performance at higher current density ..................................... 163 5.3 Investigation of Reaction Layer Exfoliation ............................................................... 168 5.4 Optimization of the LNRR Durability ........................................................................ 173 5.5 Improving the LNRR Performance with Cathode Modifications ............................... 177 5.6 Conclusion .................................................................................................................. 185 Chapter 6 Conclusions and Recommendations .......................................................................186 6.1 Conclusions ................................................................................................................. 186 6.2 Recommendations for Future Research ...................................................................... 189 Bibliography ...............................................................................................................................194 Appendices ..................................................................................................................................206 Appendix A Supplementary Information for Chapter 1 ......................................................... 206 Appendix B Supplementary Information for Chapter 2 ......................................................... 222 Appendix C Supplementary Information for Chapter 3 ......................................................... 235 Appendix D Supplementary Information for Chapter 4 ......................................................... 236 Appendix E Supplementary Information for Chapter 5 .......................................................... 241 xii List of Tables Table 1.1 The Performance Metrics, Targets, and Current Level of NH3 Electrosynthesis. .......... 8 Table 1.2 Physical Properties of Aqueous and Non-aqueous Solvents for NH3 Electrosynthesis........................................................................................................................................................ 34 Table 2.1 Properties of Different Separator Materials. ................................................................. 79 Table 2.2 Physicochemical Properties of the Carbon-based Reaction Layer in the Cathode GDE........................................................................................................................................................ 82 Table 2.3 Experimental Conditions for the LNRR Experiments with the Flow Cell Electrolyzer........................................................................................................................................................ 89 Table 2.4 Preparation of Non-aqueous and Aqueous Samples for the IB Measurement ............. 95 Table 2.5 Calculation and Conversion Factors for Different Units for rNH3 in the Literature. ... 101 Table 4.1 NH4+ Flux at OCV and GP for the Cathode-fed Operation (ΔcNH4+= 1 ppm) with N112 Membrane. .................................................................................................................................. 130 Table 4.2 System Parameters and Equilibrium Constants to Evaluate the Role of NH3 Diffusion in the Electrolyzer. .......................................................................................................................... 137 Table 5.1 Comparison of Previous LNRR Studies using Flow Cell Electrolyzers or Flooded Cells with Carbon-based Cathodes. ..................................................................................................... 184 Table A.1: Summary of Previous Aqueous ENRR Studies Performed in Non-separated (NS) Cells...................................................................................................................................................... 207 Table A.2: Summary of Previous Aqueous ENRR Studies Performed in Liquid-Liquid (L-L) Separated Cells............................................................................................................................ 208 xiii Table A.3: Summary of Previous Aqueous ENRR Studies Performed in Gas-Liquid (G-L) Separated Cells............................................................................................................................ 211 Table A.4: Summary of Previous Aqueous ENRR Studies Performed in Gas-Gas (G-G) Separated Cells. ........................................................................................................................................... 212 Table A.5: Detailed Comparison of Experimental Procedures in Conducting Quantitative Isotope NMR Tests for the Aqueous ENRR............................................................................................ 213 xiv List of Figures Fig. 1.1 Cost comparison to produce NH3 with the thermochemical (Haber-Bosch) or electrochemical (ENRR) process coupled with/without H2O electrolysis for the H2 feedstock. ... 6 Fig. 1.2 The NH3 electrosynthesis performances in aqueous systems reported in the literature. . 12 Fig. 1.3 Boxplot analyses with data distribution (black circles) for the ENRR performance in the literature. ....................................................................................................................................... 14 Fig. 1.4 Basic cell layouts for the NH3 electrosynthesis are classified based on the presence of a separator between electrodes and the reactant phases. ................................................................. 19 Fig. 1.5 Different reaction pathways for the adsorption, activation, and hydrogenation of N2 to form NH3. ...................................................................................................................................... 20 Fig. 1.6 Choosing the separator material for the NH3 electrosynthesis needs to consider its affinity to NH3/NH4+ products and proton conductivity. ........................................................................... 25 Fig. 1.7 The effect of Li+ incorporation to the ENRR performance. ............................................ 29 Fig. 1.8 The effects of different types and concentrations of ionic species in the aqueous electrolyte........................................................................................................................................................ 30 Fig. 1.9 ENRR experiments conducted in non-aqueous electrolyte. ............................................ 33 Fig. 1.10 Optimizing the pH level in aqueous systems................................................................. 36 Fig. 1.11 Use of non-aqueous solvents for the ENRR. ................................................................. 38 Fig. 1.12 Mechanistic understanding and evidence for Li-mediated non-aqueous NH3 electrosynthesis. ............................................................................................................................ 41 Fig. 1.13 Understanding of the Li-mediated non-aqueous ENRR from a step-wise perspective. 43 xv Fig. 1.14 Surface modifications by size, shape, and porosity to synergically enhance the performance of ENRR. ................................................................................................................. 46 Fig. 1.15 A microkinetic model to study the effects of electrode dimension to the ENRR. ........ 48 Fig. 1.16 Effects of catalyst layer substrate to the ENRR in aqueous electrolyte. ....................... 51 Fig. 1.17 Hydrophobicity engineering on the electrode surface improves the ENRR performance........................................................................................................................................................ 53 Fig. 1.18 The effects of operating temperature on the NH3 electrosynthesis. .............................. 56 Fig. 1.19 The effects of operating pressure on the NH3 electrosynthesis. .................................... 58 Fig. 1.20 Effects of gas flow rate to the ENRR performance. ...................................................... 61 Fig. 1.21 Recirculated electrolyte containing electrocatalysts as a design strategy to improve the ENRR performance. ...................................................................................................................... 62 Fig. 2.1 Physical characterization of the Au/KB catalyst ink used for the cathode catalyst layer........................................................................................................................................................ 67 Fig. 2.2 Physical characterization of the Au/KB cathode GDE. ................................................... 68 Fig. 2.3 A: Pt loading on the Nafion membrane measured by XRF. B: The frame used to cover the Nafion substrate for the spray coating process. Dimension unit: mm. ......................................... 69 Fig. 2.4 Characterization of MoS2 on the cathode GDE prepared from the hydrothermal synthesis process........................................................................................................................................... 70 Fig. 2.5 Elemental analysis for the MoS2 deposited onto carbon paper substrate as the cathode GDE. ............................................................................................................................................. 71 Fig. 2.6 Exploded view of the prismatic electrolyzer cell containing a catalyst-coated membrane (CCM) for the anode and a gas diffusion electrode (GDE) for the cathode. ................................ 72 Fig. 2.7 Illustrations of the liquid and gas flow configurations in the electrolytic system. .......... 73 xvi Fig. 2.8 X-ray diffraction results for different carbon-based materials used for the cathode reaction layer............................................................................................................................................... 83 Fig. 2.9 Electrolyzer cell designs for the LNRR studies in non-aqueous electrolyte. .................. 85 Fig. 2.10 Schematic of the flow cell electrolyzer. ........................................................................ 85 Fig. 2.11 Image of the flow cell electrolyzer. ............................................................................... 86 Fig. 2.12 Images of the assembly of the flow cell electrolyzer. ................................................... 87 Fig. 2.13 Anodic THF oxidation and the subsequent cationic polymerization.210 ....................... 88 Fig. 2.14 A: cross-section view of a commonly used liquid-flooded single-compartment cell where the CA, AN, and RE are all submerged in the liquid electrolyte. B: the glass single-compartment cell, which was used in the Ar-filled glove box. ........................................................................... 90 Fig. 2.15 Calibration of the pseudo-Au reference electrode versus the Li+/Li equilibrium potential (ELi+/Li = −3.04 VRHE). ................................................................................................................... 91 Fig. 2.17 Calibration for the measurement of NH4+ based on the indophenol blue method. ....... 93 Fig. 2.18 Typical IB calibration for the aqueous and non-aqueous samples. ............................... 97 Fig. 2.19 Calibration for the measurement of N2H4 based on the Watt-Chrisp method. .............. 99 Fig. 2.20 Calibration for the measurement of NO3− by the direct spectrophotometry scanning between 200 – 225 nm. ................................................................................................................. 99 Fig. 2.21 Calibration for the measurement of NO2− by the colorimetry technique. ................... 100 Fig. 2.22 Typical UV-Vis spectra for the analyses of N-containing compounds in the electrolyte as possible NH4+ oxidation intermediate or reaction before and after the electrolysis test. ....... 100 Fig. 3.1 A: LSV comparison (10th cycle) for the Au/KB GDE under Ar or N2 atmosphere. B: j – t curve for the ENRR experiments with N2 and control test with Ar. ........................................... 104 xvii Fig. 3.2 NH3 contents in the electrolyte and acid absorber from the LSV and ENRR experiments with the Au/KB GDE. ................................................................................................................. 105 Fig. 3.3 A: LSV comparison (10th cycle) for the MoS2 GDE under Ar or N2 atmosphere. B: j – t curve for the ENRR experiments with N2 and control test with Ar. ........................................... 107 Fig. 3.4 NH3 contents in the electrolyte and acid absorber from the LSV and ENRR experiments with the MoS2 GDE. ................................................................................................................... 107 Fig. 3.5 Long-term ENRR electrolysis with the MoS2 GDE at two different current density ranges...................................................................................................................................................... 108 Fig. 3.6 NH3 production rates (rgeo) and Faradaic efficiencies (FE) for the MoS2 GDE in two sets of long-term electrolysis experiments at constant current densities. .......................................... 110 Fig. 4.1 Schematics of electrochemical cells with a CEM as the separator.. .............................. 112 Fig. 4.2 NH4+ crossover in 0.05 M H2SO4 in the flow cell electrolyzer using Nafion N112 as the separator. ..................................................................................................................................... 118 Fig. 4.3 NH4+ crossover in 0.1 M NaOH in the flow cell electrolyzer using Nafion N112 as the separator. ..................................................................................................................................... 120 Fig. 4.4 NH4+ captured by the acid absorbers in the exhaust from both the cathode and anode. 6 and 25 mL 0.5 M H2SO4 were added to the first and second acid absorbers, respectively. Nafion 112 was used as the separator. .................................................................................................... 121 Fig. 4.5 NH4+ adsorption in and desorption from the N112 membrane. .................................... 122 Fig. 4.6 The NH4+ mass gain or loss in the catholyte, anolyte, and acid absorbers (bars) and the overall mass balance (lines and symbols) for the crossover tests under OCV and GP conditions...................................................................................................................................................... 123 xviii Fig. 4.7 NH4+ crossover tests with NH4+-containing DI H2O and blank Na2SO4 in separated electrolyte compartments using Nafion N112 as the separator. ................................................. 125 Fig. 4.8 1-ppm NH4+ crossover in 0.1 M Na2SO4 (ca. pH 6) using Nafion N112 as the separator. The MEA was conditioned initially in blank 2 M Na2SO4 followed by equilibrating in blank 0.1 M Na2SO4.................................................................................................................................... 126 Fig. 4.9 NH4+ desorption from contaminated N112 membrane in blank Na2SO4. ..................... 131 Fig. 4.10 Changes to the cCA (filled symbols) and cAN (empty symbols) normalized to the tested cNH4+ for the NH4+ crossover with N112 in the cathode-fed operation. ..................................... 133 Fig. 4.11 Electrolyte pH changes with different cNH4+, j, and feed locations for the Nafion N112...................................................................................................................................................... 134 Fig. 4.12 Changes to cCA (filled symbols) and cAN (empty symbols) for NH4+ crossover with N112 in anode-fed operation. ............................................................................................................... 139 Fig. 4.13 Changes to cCA (filled symbols) and cAN (empty symbols) for NH4+ crossover with N112 in symmetric fed operation. ........................................................................................................ 141 Fig. 4.14 Electrolyte pH changes with different cNH4+, j, and feed locations for the Nafion N112...................................................................................................................................................... 143 Fig. 4.15 Comparison of NH4+ recovery efficiency using the N112 as the separator at different operation conditions. ................................................................................................................... 145 Fig. 4.16 Effects of nitrogenous contaminants in the MEA with N112. .................................... 147 Fig. 4.17 Electrolysis conditioning improves the consistency in assessing the NH4+ crossover for N112 membrane. ......................................................................................................................... 148 Fig. 4.18 Comparison of NH4+ recovery efficiency for 10 ppm at −10 A m−2 using the N112 membrane that is unconditioned (H+-saturated) or conditioned with electrolysis. ..................... 149 xix Fig. 4.19 Electrolysis conditioning at −10 A m−2 for different separator materials. .................. 150 Fig. 4.20 The poor ionic conductivity of Celgard C3400 and C3500 due to the hydrophobic surface...................................................................................................................................................... 151 Fig. 4.21 1-ppm NH4+ crossover in 0.1 M Na2SO4 (ca. pH 6) using Nafion N350 as the separator. The MEA was conditioned initially in blank 2 M Na2SO4 followed by equilibrating in blank 0.1 M Na2SO4.................................................................................................................................... 153 Fig. 4.22: RENH4+ for different separator materials in cathode-fed operation under different j.. 154 Fig. 4.23 Changes to the cCA (filled symbols) and cAN (empty symbols) for the NH4+ crossover with AEM and microporous separators at 1 ppm. .............................................................................. 155 Fig. 5.1 Electrode potentials and LNRR performances for different cathode reaction layers for the constant j operation. .................................................................................................................... 165 Fig. 5.2 Images of the C45 GDEs (Nafion as binder) after the LNRR at −5 mA cm−2 with 0 or 0.5 vol% EtOH addition. The electrolyte-facing side is shown. ....................................................... 166 Fig. 5.3 Performance degradation with continuous electrolysis at −5 mA cm−2 and SEM images of the C45 cathode after the exfoliation of the liquid-facing RL. ................................................... 168 Fig. 5.4 Half-cell evaluation of the Li deposition on a Pt wire as the WE in electrolyte with and without 0.5 vol% EtOH or filtration. .......................................................................................... 170 Fig. 5.5 Images of the lithium deposition during the chronopotentiometric electrolysis for 30 min in chronological order. ................................................................................................................ 171 Fig. 5.6 Images of the lithium deposition during the chronopotentiometric electrolysis for 30 min in chronological order. ................................................................................................................ 172 xx Fig. 5.7 Electrode potentials for the LNRR tests conducted at −5 mA cm−2 with j cycling (1 min electrolysis followed by 1-min OCP resting) using different electrocatalysts and volume percentages of EtOH. .................................................................................................................. 174 Fig. 5.8 Sections of the LNRR tests with j cycling at −5 mA cm−2. ........................................... 175 Fig. 5.9 Comparison for the LNRR performance at −5 mA cm−2 using GDEs spray-coated with Pt/XC, XC, and C45 reaction layers. .......................................................................................... 176 Fig. 5.10 Results for the j cycling operation using the C45 GDE and 0.5 vol% EtOH in the electrolyte. ................................................................................................................................... 177 Fig. 5.11 EIS results for the C45 GDE (Nafion as binder) at OCP at the beginning and end of test. A: j cycling at −5 mA cm−2; B: j cycling at −3 mA cm−2. .......................................................... 179 Fig. 5.12 LNRR performances for the modified cathodes: SFG graphite replaces amorphous C45 carbon black; and/or PVDF replaces Nafion as the binder. ........................................................ 180 Fig. 5.13 Schematic of N2 mass transfer through GDEs. ........................................................... 182 Fig. 6.1 Schematic of the interactions between the N112 membrane and the NH4+ transport in different electrolyte solutions. ΔcNH4+ = 1 ppm is described. ..................................................... 187 Fig. B.1 Flow configuration with images of hardware components. .......................................... 222 Fig. B.2 The gas pressure recorded at the outlet of the cathode and the anode, as well as the inlet of the 1st acid absorber (i.e., overall back pressure at the tee connector before the exhaust gas enters the acid absorbers). ..................................................................................................................... 223 Fig. B.3 CV scans of an Au GDE in the electrolyzer cell at a scan rate of 100 mV s−1 in Ar. ... 224 Fig. B.4 Images for the assembly of the prototype electrolyzer cell. ......................................... 227 Fig. B.5 Images of the prototype electrolyzer adapted from a commercial electrolyzer setup (TP5E, Tandem Technologies)................................................................................................................ 227 xxi Fig. B.6 Comparison between the prototype TP5E cell and the final bolted cell. ...................... 228 Fig. B.7 Comparison of electrolyte color change due to the availability of H+ donor. .............. 229 Fig. B.8 Images of electrolyte color change that corresponds to the UV-Vis spectroscopy shown in Fig. B.7B – D, respectively..................................................................................................... 230 Fig. B.9 Viscosity (μ) measurement for the electrolyte after LNRR experiments. .................... 231 Fig. B.10 Comparison for the cathode and anode potentials (ECA and EAN, respectively) in the 4-h LNRR using the prototype TP5E cell to study the effects of H+ source to the electrolyte degradation. ................................................................................................................................. 233 Fig. B.11 Ohmic resistance (Rs) measured by EIS at OCV using the prototype TP5E cell using different catalyst layer locations and LiBF4 concentrations. ...................................................... 234 Fig. C.1 cNH3 in the electrolyte and acid absorber after the LSV and ENRR experiments with the Au/KB GDE. ............................................................................................................................... 235 Fig. C.2 Repeated ENRR evaluation of the Au/KB GDE with controls. ................................... 235 Fig. D.1 The crossover results from Kapton polyimide film as the separator. .......................... 236 Fig. D.2 NH4+ desorption to blank 0.1 M Na2SO4 from a contaminated MEA membrane that was used for crossover tests in DI H2O. ............................................................................................. 236 Fig. D.3 The percentage of NH4+ and NH3 of the total ammonia nitrogen in relation to the electrolyte pH at room temperature and pressure. ...................................................................... 237 Fig. D.4 Remaining NH3 that escapes from the first acid absorber but is capture by the second acid absorber after the crossover experiments with N112. ................................................................. 237 Fig. D.5 Temporal pH changes for cNH4+ = 3400 ppm at −100 A m−2 for the Nafion N112 with different feed locations. .............................................................................................................. 238 xxii Fig. D.6 NH4+ captured by the 1st acid absorber (ca. 6 mL) in the exhaust from both the cathode and anode. ................................................................................................................................... 238 Fig. D.7 SEM images of as-received Celgard diaphragms with different magnifications. ........ 239 Fig. D.8 Process of coat wet out for C5550. ............................................................................... 240 Fig. E.1 SEM images of the freshly prepared GDEs. ................................................................. 241 Fig. E.2 SEM images of the electrode surfaces after the LNRR experiments at constant j of −5 mA cm−2 ............................................................................................................................................. 242 Fig. E.3 Energy-dispersive X-ray spectroscopy (EDX) measurement of the electrolyte-facing side of the C45 cathode after electrolysis with 0 vol% EtOH. ........................................................... 243 Fig. E.4 EDX measurement of the electrolyte-facing side of the C45 cathode after electrolysis with 0 vol% EtOH. .............................................................................................................................. 244 Fig. E.5 EDX measurement of the electrolyte-facing side of the Pt/XC anode after electrolysis with 0 or 0.5 vol% EtOH. .................................................................................................................. 245 Fig. E.6 Cyclic voltammetry (CV) scans in 1 M LiBF4 using a Pt wire as the working electrode in the single-compartment glass cell. .............................................................................................. 246 Fig. E.7 Comparison for the LNRR performance at −5 mA cm−2 using GDEs spray-coated with Pt/XC, XC, and C45 catalysts. .................................................................................................... 246 Fig. E.8 Long-term LNRR tests with periodic sampling for a total of 17.5 h operation duration for the C45 GDE (Nafion as binder). ............................................................................................... 247 Fig. E.9 Control experiments at OCP with the C45 GDE (Nafion as binder) and 0.5 vol% EtOH addition for 4 h. ........................................................................................................................... 247 xxiii List of Symbols 𝐴𝑔𝑒𝑜 Geometric surface area, cm−2 𝐶𝑑𝑙̅̅ ̅̅ Specific Double Layer Capacitance, μF cm−2 𝐶𝑑𝑙 Double Layer Capacitance, μF 𝑐𝐶𝐴 / 𝑐𝐴𝑁 NH4+ Concentration in the Catholyte/Anode, ppm 𝑐𝑁𝐻4+𝐷𝑒𝑠𝑜𝑟𝑏 Desorbed NH4+ from Contaminated Nafion Membranes, ppm 𝑐𝑁𝐻3 NH3 Concentration, ppm 𝑐𝑁𝐻4+ / 𝑐𝑎𝑐𝑖𝑑 NH4+ Concentration in the Electrolyte/Acid Absorber, ppm Δ𝑐𝑁𝐻4+ NH4+ Concentration Gradient between Catholyte and Anolyte, ppm 𝐷𝑒,𝑁𝐻3 Effective Diffusivity of NH3, m2 s−1 𝐸𝐸 Energy Efficiency, % 𝐸𝑐𝑒𝑙𝑙 Cell Voltage, V 𝐸𝐶𝐴 / 𝐸𝐴𝑁 Cathode/Anode Potential, VAu 𝐸𝑒𝑞° Equilibrium Potential, VRHE 𝐸𝐿𝑖+/𝐿𝑖 Equilibrium Potential for Li Deposition/Stripping, VRHE 𝐹𝐸 Faradaic Efficiency, % 𝑖𝑐 / 𝑖𝑎 Peak Cathodic/Anodic Current Density, mA cm−2 𝑗 Current Density, mA cm−2 or A m−2 𝐽𝑁𝐻3𝐷𝑖𝑓𝑓 Flux of Diffused NH3 across the Separator, mol m−2 𝐽𝑁𝐻3𝐸𝑣𝑎𝑝 Flux of Evaporated NH3, mol m−2 𝐽𝑔𝑎𝑠 Inert Gas Flux Aerating the Electrolyte, mol m−2 𝑗𝑁𝐻3 Current Density toward NH3, mA cm−2 or A m−2 𝐾𝐻,𝑁𝐻 Henry’s Constant for NH3, mM bar−1 𝐿 Separator Thickness, μm 𝑝𝐾𝑎 Acid Dissociation Constant 𝑝𝑡𝑜𝑡 Total Gas Pressure, bar xxiv 𝑄𝑑𝑒𝑝𝑜𝑠𝑖𝑡 / 𝑄𝑠𝑡𝑟𝑖𝑝 Charge for Li Deposition/Stripping, C 𝑅𝐸𝑁𝐻4+ Recovery Efficiency of NH4+, % 𝑟𝑁𝐻3,𝑔𝑒𝑜 / 𝑟𝑁𝐻3 Area Specific NH3 Production Rate, mol s−1 cm−2 or μmol h−1 cm−2 𝑣 Voltammogram Scan Rate, mV s−1 𝑉𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑡𝑒 / 𝑉𝑎𝑐𝑖𝑑 Volume of the Electrolyte/Acid Absorber, mL Greek Π Ratio between Diffused and Evaporated NH3 Fluxes 𝜙 Electric Field 𝜂 Charge Efficiency, % xxv List of Abbreviations AEM Anion-Exchange Membrane AN Anode CA Cathode CCM Catalyst-coated Membrane CE Counter Electrode CEM Cation-Exchange Membrane CP Chronopotentiometry CP/CC Carbon Paper/Cloth CV Cyclic Voltammetry DFT Density Functional Theory ECSA Electrochemical Surface Area EDX Energy-Dispersive X-ray Spectroscopy EIS Electrochemical Impedance Spectroscopy ENRR Electrochemical Nitrogen Reduction Reaction EtOH Ethanol GDE Gas-Diffusion Electrode GDL Gas-Diffusion Layer G-G Gas-gas G-L Gas-liquid GP Galvanostatic Polarization HER Hydrogen Evolution Reaction HOR Hydrogen Oxidation Reaction IEC Ion Exchange Capacity, meq g−1 IL Ionic Liquid IPA Isopropyl Alcohol KB Ketjenblack LIB Lithium Ion Battery xxvi L-L Liquid-liquid LNRR Lithium-mediated Nitrogen Reduction Reaction LSV Linear Sweep Voltammetry MEA Membrane Electrode Assembly NMP N-Methyl-2-pyrrolidone NMR Nuclear Magnetic Resonance NS Non-separated OCP Open Circuit Potential OCV Open Circuit Voltage OER Oxygen Evolution Reaction PEEK Polyetheretherketone PP Potentiostatic Polarization ppm part-per-million PTFE Polytetrafluoroethylene PVDF Polyvinylidene Fluoride RDE Rotating Disk Electrode RE Reference Electrode RHE Reversible Hydrogen Electrode RL Reaction Layer SAED Selected Area Electron Diffraction sccm Standard Cubic Centimeters per Minute SEI Solid Electrolyte Interface SEM Scanning Electron Microscopy SSC Stainless Steel Cloth TEM Transmission Electron Microscopy THF Tetrahydrofuran WE Working Electrode XC Vulcan XC72R XRD X-ray Diffraction XRF X-ray Fluorescence xxvii Acknowledgements I would like to express my deepest gratitude to my supervisors Prof. David Wilkinson and Prof. Elöd Gyenge, for their mentorship, patience, and guidance throughout my entire Ph.D. program. The work experience in their groups is absolutely nurturing and inspiring, and I will always be grateful for the opportunity to pursue my Ph.D. study with them in Vancouver. I would also like to thank my committee member Prof. Vikram Yadav who has taught me not only during my post-graduate studies but also ever since I entered the field of chemical engineering. My appreciation also goes to my committee member Prof. Edouard Asselin, who helps me greatly in achieving the degree. I would also like to acknowledge the funding opportunities provided by the NRC Ideation Program and the Challenge Program, and the NSERC NanoMat Program. I feel so privileged to be surrounded by outstanding and enthusiastic fellow graduate and undergraduate students in the Wilkinson and Gyenge groups. Conversations with them are always enlightening. Many thanks to the advice and assistance from research associates, Dr. Baizeng Fang, Dr. Arman Bonakdarpour, and Dr. Ivan Stosevski. Interactions with them gave me valuable perspectives through the lens of senior scholars. Lastly, I would like to acknowledge my whole family for their relentless support, mentally and physically, that keeps me always in high spirits even when struggling with difficulties in research and in life. My caring mother, Xiaomei Zhang, has been and will be regarded as a role model for me. My partner, Xiao Han, has supported me selflessly despite adversities and I hope our life-long companionship has only just started. xxviii Dedication Dedication to my beloved family, friends, and things that make us happy and free. 路阻且长，行则将至； 行而不辍，未来可期。 1 Chapter 1 Introduction 1.1 Research Objectives and Thesis Layout Objective: the goal of this thesis is to show the importance of electrolyzer cell design， operation, and measurement on the aqueous and non-aqueous electrochemical N2 reduction reaction (ENRR). The following aspects were investigated in-depth with corresponding approaches: a. Assess the baseline performance of model electrocatalysts in flow cell electrolyzers to verify the feasibility of aqueous ENRR. Approach: (1) Under aqueous conditions, the severity of false positives was evaluated with the source of contaminants being identified. (2) A set of rigorous pre-treatment protocols are proposed to remove adventitious contaminants from commercial and research electrodes and/or membranes. b. Evaluate the reliability of separator materials against product crossover for accurate ENRR performance assessment. Approach: (1) The reliability of the most used separator, the Nafion membrane, was assessed in a prismatic flow cell under different electrolyte environment and operating conditions. (2) Alternative separators inert against NH4+ adsorption and crossover were explored for consistent ENRR product accumulation and accurate quantification. c. Conduct the non-aqueous ENRR using Li as the reaction mediator with a GDE configuration as the cathode design to generate true positives of NH3 electrosynthesis. Approach: (1) Under non-aqueous conditions, the sacrificial decomposition of the electrolyte (EtOH and/or THF) was prevented in a customized three-compartment electrolyzer cell by utilizing the anode HOR to sustain the H+ demand for the cathode Li-mediated ENRR. (2) Cathode GDEs were prepared with carbon-based reaction layers as a 2 low-cost alternative to precious metal-based GDEs reported in the literature. (3) the ENRR performance was further improved by modifying the carbon or binder material in the reaction layer, electrolyte composition, and electrolysis operating conditions. Thesis layout: Chapter 1 provides an in-depth introduction to NH3 electrosynthesis in both aqueous and non-aqueous electrolyte, with emphases on the cell design and component selections reported in the literature to date. Drawbacks including the prevalence of false positives, low product yields in aqueous ENRR studies, and poor energy efficiencies and operational stability in non-aqueous ENRR studies are discussed. This information determines the scope of this study and methodology toward the improvement of the electrolyzer cell design, operation, and measurement. Chapter 2 describes the experimental procedures and setups for the NH3 electrosynthesis under aqueous and non-aqueous conditions. Electrode preparation methods, physical and electrochemical characterization techniques, and experimental protocols are also included. Chapter 3 shows results from preliminary aqueous ENRR experiments using Au and MoS2 as the cathode electrocatalysts in the flow cell electrolyzer. With control experiments, the positive detection of NH3 appears to be a false positive due to nitrogenous contaminants. Chapter 4 systematically investigates the interaction between NH4+ in the electrolyte and different separator materials under various operation conditions. As Section 1.4.1 will introduce, the Nafion membrane, the predominant choice of separator used in aqueous cells, has been questioned as to whether it is an inert and non-permeable separator to NH3/NH4+. Exploring this potential issue improves the reliability of separator selection for the aqueous ENRR electrolyzer design. Chapter 5 demonstrates the production of NH3 in non-aqueous electrolyte using Li as the mediator in a customized three-compartment cell. Operating conditions and electrolyte composition are 3 varied to achieve the optimal activity and selectivity toward the ENRR with a carbon-based reaction layer for the cathode. The operational durability becomes a concern for high current density operation with observation of reaction layer exfoliation. A current-cycling approach were adopted along with modifications to the reaction layer composition, which enhances the NH3 production performances and improves the operational durability to over 8 h, respectively. Chapter 6 concludes this dissertation and offers suggestions for future work that can be investigated based on the findings of this research. 1.2 General Overview of Ammonia Electrosynthesis As the second-largest chemical produced globally, the annual supply of NH3 has reached over 180 million metric tonnes with a market value of US$67 billion.1,2 Nearly 80% of the NH3 supply is devoted to manufacturing nitrogen-based fertilizers, which takes the central stage in securing food supply in many countries. As the global population (7.8 billion in 2020) is projected to reach 9.5 billion by 2050, the demand for NH3 is also expected to increase to as much as 400 million tonnes per year.2,3 Meanwhile, if the currently estimated compound annual growth rate of 6.4% continues, the market value for NH3 will also expand to US$430 billion in 2050, offering remarkable economic incentives to innovate NH3-related technologies. In fact, beyond the agricultural application, NH3 has drawn increasing attention as a potential energy carrier in clean energy applications. Specifically, as a carbon-free fuel, NH3 has found multiple uses in fuel cells and direct-combustion gas turbines as a single- or co-fuel. These applications are expected to proliferate with improved emission abatement to limit the nitrogen oxides in the exhaust gas.4 More importantly, with a high energy density (13.6 GJ m–3 when liquified at 10 bar), NH3 outperforms many renewable energy carriers such as hydrogen (H2) in terms of NH3’s suitability 4 for long-term storage and long-range transportation.5 Therefore, this makes NH3 very competitive in integrating renewables while mitigating the intermittent nature of these energy supplies.6 For over a century, the rising demand for NH3 has been provided by the Haber-Bosch process which is increasingly deemed unsustainable amid intense energy and environmental costs. In the Haber-Bosch process (Equation 1.1), nitrogen (N2) and H2 are catalytically converted into NH3 on a heterogeneous surface at approximately 700 K and 100 bar.7 To derive the H2 feedstock, hydrocarbons such as natural gas, heavy oil, and coal are often consumed through the steam reforming process while emitting carbon dioxide as a by-product.8 As a result, the Haber-Bosch process consumes roughly 2% of annual global energy and discharges over 300 million metric tonnes of greenhouse gases every year.9 Additionally, as the Haber-Bosch process requires chemical looping to elevate the single-pass conversion from a mere 15% to above 98%, a continuous operation with sophisticated operation control and optimization has to be established, thus demanding intensive capital investments and high operational costs.10,11 Hence, the Haber-Bosch process usually operates at centralized locations near H2 precursor (e.g., natural gas) sources to achieve economies of scale and to satisfy the need for continuous feedstock supply, which consequently restricts the deployment of intermittent renewables as energy sources and leads to inequality in NH3 access for remote areas. N2(g) + 3H2(g) ↔ 2NH3(g) ∆G°298 K = –16.5 kJ molNH3–1 (1.1) N2(g) + 6H+(aq.) + 6e– ↔ 2NH3(aq.) E°298 K = 0.092 VSHE at pH = 0 (1.2a) N2(g) + 6H2O(l) + 6e– ↔ 2NH3(aq.) + 6OH–(aq.) E°298 K = −0.736 VSHE at pH = 14 (1.2b) Due to the high environmental and infrastructure costs of the Haber-Bosch process, researchers are developing sustainable and modular alternatives to transform the NH3 manufacturing industry. 5 NH3 electrosynthesis via the electrochemical nitrogen reduction reaction (ENRR, Equation 1.2a and 1.2b) represents an attractive option with several notable benefits: 1) The NH3 electrosynthesis is driven by electricity in the form of electrochemical potentials and is thermodynamically feasible at room temperature and pressure, which avoids the stringent operating conditions and high capital costs that an Haber-Bosch plant often needs, hence offering operational flexibility and affordability.12 2) ENRR can employ protons (H+) directly for the hydrogenation of N2 without the necessity of reforming hydrocarbons into H2 as feedstock. Specifically, by employing an aqueous liquid electrolyzer, H+ from water (H2O) or the hydronium ion (H3O+) can reduce N2 to NH3 via proton-coupled electron transfer without explicit involvement of H2.13,14 The abundant proton sources available for NH3 electrosynthesis, therefore, incentivize R&D activities in this area especially for the niche deployment at the point of use (or so-called “islanded” use). 3) Renewable electricity derived from solar, wind, or hydro energy can be directly coupled with NH3 electrosynthesis to mitigate the environmental cost of the process. Although renewables can be used in water electrolysis to generate H2 feedstock for the Haber-Bosch process to potentially alleviate the associated carbon footprint, additional infrastructure for H2 storage and handling is required to satisfy the significant and continuous H2 demand. Additionally, as compared in Fig. 1.1,a at a lower production scale for the islanded use (e.g., 30 kg day–1), the ENRR process shows significant cost reduction than cases involving the Haber-Bosch process in capital investment and operation & maintenance costs.11 Therefore, the direct electrosynthesis of a Assuming a levelized electricity cost of USD$0.03 kWh–1, an intrinsic exchange current density of 10–7 A m–2, an operating current density of 0.2 A m–2, and a Faradaic efficiency of 30% under operation conditions of 100 °C and 5 atm. The social cost of CO2 is USD$291 tonCO2–1 and the emission from the Haber-Bosch is equivalent to 1.5 tonCO2–1 per tonneNH3–1. A CO2 emission of 10 gCO2 kWh–1 is assumed from the renewable energy application. 6 NH3 is attractive to rid reliance on economies of scale and achieve environmentally benign operation. Currently, inputting green H2 as the feedstock to the Haber-Bosch process (Gen 2 NH3 as termed by MacFarlane et al.6) has been envisioned and commercially realized in many countries in bridging the transitioning from the Haber-Bosch process (Gen 1 NH3) to the electrochemical NH3 production (Gen 3 NH3). Such an intermediate approach obtains the electrochemically produced H2 either by water electrolysis at (near) ambient temperature and pressure using renewable electricity, or by solid-oxide electrolysis at high temperatures utilizing the waste heat released from the exothermic Haber-Bosch reactor.15,16 In the short-term by approximately 2030, Gen 2 has the potential to complement the Haber-Bosch process with significantly reduced carbon emissions, providing the commercially available mega-watt-scale water electrolyzers that are capable of achieving high current densities (>1 A cm–2) and high energy efficiencies (>75%).6 Nevertheless, Fig. 1.1 Cost comparison to produce NH3 with the thermochemical (Haber-Bosch) or electrochemical (ENRR) process coupled with/without H2O electrolysis for the H2 feedstock. A: the case for a production scale of 2000 ton NH3 day-1. B: the case for a production scale of 30 kg NH3 day-1. Reproduced from Fernandez and Hatzell11 with permission. Copyright 2020, The Electrochemical Society. 7 high capital investment, availability of renewable energy, and centralized production modes remains to be addressed for the Gen 2 NH3 production, which in turn is making the ENRR as a more attractive alternative to the Haber-Bosch process in the long term. Comparisons regarding those aspects are provided in the following: (1) Firstly, the Gen 2 NH3 production requires expensive electrolyzer modules (estimated as USD$1 million per MW) on top of the costs of Haber-Bosch plants, which increases the price of produced NH3 by ca. USD$800 tonneNH3–1 from USD$1000 – 1500 tonnesNH3–1 depending on the plant size.16,17 By contrast, the Gen 3 NH3 production requires the expenditure only of the ENRR electrolyzer in the capital investment. Additionally, the catalyst used for the Haber-Bosch reactor has low tolerance to O2 impurities in the N2 stream at ppm levels but O2 could be electrochemically reduced on the cathode to H2O which is unlikely to be detrimental to the electrocatalytic stability of the electrode. Therefore, the ease of N2 separation from air could provide additional capital and operational cost reduction compared to the Gen 1 and Gen 2 NH3 production that remain associated to the Haber-Bosch process. (2) Secondly, maintaining the high temperature and pressure operation of the Haber-Bosch reactor requires a continuous supply of H2 feedstocks. However, Gen 2 NH3 production incorporating renewable electricity is often affected by the intermittent nature of many renewable sources such as solar and wind. Cryogenic or pressurized H2 storage or stationary energy storage (i.e., batteries) technologies are therefore necessary to ensure uninterrupted H2 supplies, which inevitably incurs expensive equipment and potential safety concerns over H2 handling/storage and grid integration. For the ENRR, in theory, could generate NH3 intermittently depending on the availability of electricity from solar or wind particularly for the aqueous environment and ambient conditions. 8 (3) Lastly, at the core of Gen 2 NH3 production, adding an industrial level electrolytic system compatible to the H2 demand of the Haber-Bosch process only consolidates the reliance of economies of scale and increases the imparity of fertilizer availability in rural areas. On the contrary, a modular ENRR electrolyzer could potentially lead to decentralized NH3 production with a point-of-use application mode.16 However, despite being demonstrated to be economically and environmentally beneficial by many techno-economic models, NH3 electrosynthesis is only considered favourable over the Haber-Bosch process when assuming its production rate and efficiency are at a level of practical relevance. As shown in Table 1.1, Giddey et al.18 and the U.S. Department of Energy19 both provided performance targets for NH3 electrosynthesis to acquire commercial competitiveness. Nevertheless, most experimental studies in aqueous systems did not provide satisfactory performance in the specific production rate (rNH₃,geo), Faradaic efficiency (FE), or energy efficiency (EE) toward NH3 production, highlighting deficiencies of this emerging technology in activity, selectivity, and energy cost. For example, rNH₃,geo and current densities (jNH₃) reported from most NH3 electrosynthesis experiments are at least two orders of magnitude lower than the proposed targets. Due to such low ENRR performances, existing studies have only obtained trace amounts of NH3 from small-scale electrolyzers, which are not only challenging to be reliably quantified but also highly susceptible to background contaminants that contributed to falsely concluded results. Table 1.1 Performance Metrics, Targets, and Current Level of NH3 Electrosynthesis. Performance (unit) Target from 18 Target from 19 Current levela rNH₃,geo (mol s–1 cmgeo–2) > 10–7 > 10–6 < 10–9 jNH₃ (mA cm–2) > 30 > 300 < 0.3 FE > 50% > 90% < 30% 9 a Data collected for the aqueous ENRR only, including studies with or without isotope-labelled experiments for performance validation (see Appendix A for details). Theoretical models have provided mechanistic insights to explain bottlenecks in promoting activities and efficiencies in NH3 electrosynthesis.13,20–23 Firstly, N2 is chemically stable with strong triple-bond energy of 941 kJ mol–1, and therefore completely dissociating the N≡N bond is extremely difficult at ambient conditions. Cleaving just the first bond demands an overwhelming energy input of 410 kJ mol–1. Secondly, N2 is electronically stable due to high ionization energy (1525 kJ mol–1) and a large energy gap (1044 kJ mol–1) between the highest occupied and lowest unoccupied molecular orbitals. Therefore, the electron transfer to N2 molecules is kinetically difficult with a very negative equilibrium potential (Equation 1.3). Even by coupling with the proton transfer (Equation 1.4), the activation of N2 still requires a relatively large electric potential (–3.2 VRHE) to overcome the inherent kinetic barrier.24 Thirdly, N2 has a lower proton affinity than ENRR intermediates such as diazene (N2H2) and hydrazine (N2H4), hence making the initial N2 protonation a more rate-determining step than the subsequent ones (Equation 1.5 – 1.7).13 Last but not least, as most ENRR tests were conducted in aqueous conditions, excessive protons from H2O or H3O+ can be electrochemically reduced to H2 with faster kinetics (Equation 1.8), which severely reduces the FE of the ENRR. Skúlason et al.20 used a density functional theory (DFT)-based electrochemical model to demonstrate that overcoming the kinetic disadvantage of ENRR requires a more negative overpotential of at least 0.4 V than the hydrogen evolution reaction (HER) on flat or stepped transition metal surfaces. Moreover, for transition metals that are predicted to protonate N2 most actively (e.g., Mo and Fe), H atoms bind more strongly with the heterogeneous surface EE — > 60% < 30% 10 instead of N atoms at the same overpotential, which leaves fewer available catalytic sites to bind and activate N2, eventually making the ENRR even less efficient.20,21 N2 + e– ↔ N2– E°298 K = –4.16 VRHE (1.3) N2 + H+ + e– ↔ N2H E°298 K = –3.20 VRHE (1.4) N2 + 2H+ + 2e– ↔ N2H2 E°298 K = –1.19 VRHE (1.5) N2H2 + 2H+ + 2e– ↔ N2H4 E°298 K = 0.529 VRHE (1.6) N2H4 + 2H+ + 2e– ↔ 2NH3 E°298 K = 0.939 VRHE (1.7) 2H+(aq.) + 2e– ↔ H2(g) E°298 K = 0 VSHE at pH = 0 (1.8a) 2H2O(l) + 2e– ↔ 2OH–(aq.) + H2(g) E°298 K = –0.828 VSHE at pH = 14 (1.8b) Currently, strategies to solve the activity and selectivity challenges in NH3 electrosynthesis have been largely relying on the development of ENRR electrocatalysts. In this regard, multiple reviews have summarized approaches and rationales to guide the electrocatalyst design and synthesis.13,24–26 However, the current electrocatalyst-focused strategy has not been able to elevate the rate and scale of NH3 production from electrolysis to an industrially relevant level. A thorough literature search leads to the data collection visualized in Fig. 1.2A – B. Different shapes of the symbols (triangle, square, circle, and star) correspond to the non-separated (NS) cell, liquid-liquid (L-L) separated cell, gas-liquid (G-L) separated cell, and the gas-gas (G-G) separated cell, respectively, which will be illustrated in Fig. 1.4. Different patterns of the symbols (empty, crossed, and filled) correspond to studies without nuclear magnetic resonance (NMR) tests, with qualitative NMR 11 tests, and with quantitative NMR tests, respectively, to investigate the origin of apparent NH3 products. The filled symbols are labelled with references in the Appendix A to signify the improved validity of positive results from these studies. Among those literature results, however, false positives remain possible due to the lack of rigorous cleaning protocols or experimental repeats (see Table A.5 in the Appendix A). Notably, most ENRR studies endure low specific NH3 formation rates and poor efficiencies, especially in the aqueous environment. Specifically, out of 163 aqueous ENRR studies, only 10 (ca. 6%) references achieved an excellent FE of above 30%. Similarly, out of 146 studies that reported the rNH₃,geo, merely 6 (ca. 4%) can reach an excellent level of above 10 x 10–10 mol s–1 cmgeo–2. Furthermore, the reported performances are measured mostly with respect to the geometric surface area of the electrode, instead of its real or electrochemically active surface area (ECSA). Given that the nano-sized electrocatalysts have been prevalently applied, their large surface-to-volume ratio makes it reasonable to assume much lower intrinsic ENRR rates (rNH₃,ECSA) than the apparent rNH₃,geo reported in Fig. 1.2A – B. In addition to the lack of emphasis of low intrinsic ENRR activities (i.e., low exchange current densities), the presence of outliers (e.g., Ref. 145) needs additional attention. Questions have been raised about the genuine source of nitrogen in those early works,27,28 given the use of nitrogenous precursors for the electrocatalysts that are not rigorously examined as contamination sources. 12 Fig. 1.2 The NH3 electrosynthesis performance in aqueous systems reported in the literature. A: the specific NH3 production rate normalized to the mass loading of the electrocatalyst (rm) plotted against the faradaic efficiency (FE). B: the specific NH3 production rate normalized to the geometric surface area of the cathode (rNH₃,geo) and the average ENRR current density (jNH₃) are plotted against the FE. All collected data with references are provided in the Appendix A. Note that all axes are in log-10 scale except for the linear section of the y-axis after the axis break. 13 Furthermore, the available ENRR performance results also display a prominent level of variance, even among studies that employ the same electrocatalytic material. Admittedly, such performance inconsistency can be justified by improvements of the catalyst design (e.g., facet, size-reduction, heteroatom-doping/alloying, etc.). However, considering the inconsistency in the reported ENRR data (two to three orders of magnitude in rNH₃,geo and FE) and contradiction over experimental and theoretical conclusions, simply correlating good performances in NH3 electrosynthesis with new catalyst designs needs careful examination. Firstly, the reported overall NH3 yields from the ENRR in most applications, especially in aqueous systems, often have only a small quantity that requires a thorough examination to be definitively determined as being derived from N2. Potential false positives or inaccurate measurements of genuine ENRR performances can originate from NH3 contamination or the reduction of nitrogen-containing species such nitrogen oxides (NOx), nitrate (NO3–) and nitrites (NO2–) from cell components including but not limited to the electrocatalyst, electrolyte, and N2 gas supply. In addition, some NH3 quantification techniques may provide levels of consistency and detection limits that are insufficient to the NH3 electrosynthesis system due to miniscule NH3 production rates. The commonly applied colorimetric methods (e.g., indophenol blue or Nessler’s reagent tests) are often found less reliable to detect low concentrations of NH3 (< 0.2 ppm), which are close to many reported ENRR performances in the literature.29,30 The accuracy of colorimetric and ion chromatographic approaches are also dependent on the ionic species and pH level of the analyte solution. Hence, when the NH3 concentration yielded from a single test is not significantly (at least one order of magnitude) higher than the background impurities, the reported ENRR performance can easily be biased, likely contributing to the high level of variance and even outliers among previous ENRR results (Fig. 1.3A – B).27,31–33 When the applied current density is low (e.g., 0.1 14 mA cm-2 or less), even ppm-level NH3 contamination could lead to misleadingly high 𝐹𝐸 that mathematically amplifies the influence of background contamination. Fig. 1.3 Boxplot analyses with data distribution (black circles) for the ENRR performance in the literature. A – B: rNH₃,geo (or jNH₃) and FE, respectively. For each box plot, data points are collected based on the active metal species used in the electrocatalysts. The box plots are ordered based on the number of occurrences for each metal element in our data collection. Note that this simple statistical analysis is intended to demonstrate the large variance and possible outliers (beyond the 1.5x interquartile range) in the literature. 15 Given the ambiguity in justifying the N source, rigorous testing procedures for conducting NH3 electrosynthesis experiments have been proposed,28,31,34–36 which emphasize performing robust control experiments (e.g., testing catalyst-free electrodes or switching N2 to argon gas supply), quantifying and removing N-containing impurities, and verifying genuine NH3 production from N2 based on quantitative isotope-labelled tests. With isotope-labelled 15N2 as opposed to 14N2 as the gas source, one can distinguish the genuine ENRR product (15NH3) from impurities (14NH3) based on the respective characteristic triplet and doublet peaks from 1H nuclear magnetic resonance (NMR) spectroscopy measurement, therefore validating the observed ENRR activity. The conclusion can be more convincing if the NMR-detected 15NH3 content is quantitatively cross-examined with results obtained from other analytical measurements (e.g., indophenol blue, Nessler’s reagent, and ion chromatography). One should also be warned that performing successful quantitative NMR tests does not guarantee unquestionable NH3 electrosynthesis from N2 because of insufficient purities (98 – 99% by atom) in most commercially available 15N2 gas supply. Without stringent gas cleaning protocols in place, the high levels of labile N-containing compounds (as much as 1900 ppm 15NH3) in the 15N2 gas supply can easily skew the detected NH3 from the NMR measurement.37 Therefore, rigorously validating the reported ENRR performance with NMR as the tool should firstly purify the 15N2 gas prior to the electrolysis tests to reduce the 15NOx and 15NH3 contents below detection limits or consistently low enough (one order of magnitude lower than the acclaimed 14NH3 yield) for the background subtraction. Moreover, repeating the 15N2 electroreduction to prepare multiple independent samples is encouraged to improve the experimental rigor and certainty. Unfortunately, as highlighted in Fig. 1.2A – B, qualitative NMR tests have not been widely adopted, let alone quantitatively verifying the apparent NH3 yield. Among the limited cases of studies with quantitative NMR tests, some follow certain 16 gas cleaning procedures (see Table A.5 in the Appendix A), but their efficacy in removing contaminants from the 15N2 gas supply was not analytically evaluated. To aggravate the uncertainties in isotope ENRR experiments, the positive presence of 15NH3 was rarely corroborated by repeated electrolysis experiments. Secondly, other aspects of the electrochemical system may also have significant ramifications in promoting or compromising NH3 production. Particularly, the cell design and the cell operation deserve further research as they are essential elements in realizing the rational design of electrocatalysts while avoiding possible interferences that contribute to false positives. For example, Singh et al.21 expected the ENRR to be favoured if the electron, proton, or both species can be modulated by modifying the electrode structure (e.g., applying hydrophobic or electron-insolating surface layer) or the electrolyte composition. Experimental studies have also provided evidence that improved cell components and/or interfacial designs can have significant contributions to elevating the ENRR performance under suitable operating conditions.38–43 To rectify false positives, pollution-prone cell components (e.g., electrolyte salt and separator) are being recognized,33,44 and contamination-free alternatives are proposed with the parallel execution of rigorous cell operation protocols.31,32,45 Thirdly, the electrochemical tests have mostly been conducted in laboratory scale electrolyzers with a surface area less than 5 cm2 for durations less than 4 h. Consequently, the NH3 product accumulated in the electrolyte is unlikely to reach a concentration well above the detection limit of the colorimetric method (e.g. < 0.2 ppm), and the verification of the nitrogen source (N2 or nitrogenous compounds) inevitably relies on isotope labelling experiments that are not only labor-intensive but also constrained by the available experimental apparatus. On the other hand, implementing novel electrocatalysts onto electrodes with larger surface areas (> 5 cm2) could 17 potentially elevate the overall NH3 yield for an enriched concentration (at least > 1 ppm or ideally > 10 ppm) that well exceeds the background contamination levels. Essentially, realizing the scalability of the electrolytic system during the early stage of the ENRR technology development could get rid of the expensive and time-consuming isotope labelling experiments and encourage wider engagement among scientific community members with prototypes that can be easily reproduced. Therefore, designing a reliable electrolytic cell and operating it at beneficial conditions with rigorous protocols holds promises in improving the unsatisfactory development in the field of NH3 electrosynthesis. Given the kinetic difficulties in activating and reducing N2, cell design and operation should be understood and advanced jointly along with electrocatalyst designs and operational adjustments, if not being the pre-requisite. Focusing on the importance of cell design and operation, this chapter aims to highlight the status and strategies that have or can be implemented to enhance the ENRR. Firstly, the commonly used cell configurations in the literature are reviewed and components that are of significance in improving N2 chemisorption, activation, and mass transfer are identified with attention to how to suppress the competing HER (Section 1.3). Considerations regarding critical cell components including the separator, liquid electrolyte, and electrode architecture will be specifically discussed (Section 1.4). Furthermore, experimental conditions that link strongly with the ENRR performance to achieve a reliable and stable operation are summarized (Section 1.5). Due to uncertainties from the product quantification, caution should be taken in interpreting any improvement on NH3 electrosynthesis from different design and operating strategies. Overall, by identifying gaps and opportunities in cell design and operation, this chapter offers directions to improve future research in NH3 electrosynthesis. Those gaps also formulate the research objectives that will be pursued in the remaining chapters of this thesis. 18 1.3 Basic Electrochemical Cell Layouts According to the presence of a separator to compartmentalize the cathode and the anode and the reactant phase that is in direct contact with the electrocatalysts, four basic electrolytic systems are classified: non-separated (NS), liquid-liquid (L-L) separated, gas-liquid (G-L) separated, and gas-gas (G-G) separated cells. These classifications are also indicated in the performance visualization in Fig. 1.2A – B. Firstly, NS cells are the simplest cell design to conduct NH3 electrosynthesis, which consist of a classic three-electrode setup submerged in the liquid electrolyte solution (Fig. 1.4A). The cathode (i.e., working electrode) contains ENRR-active materials to adsorb and activate the dissolved N2 in the surrounding liquid electrolyte solution (aqueous in most studies). Under ambient conditions, it is calculated to be more energetically favourable for N2 to associatively adsorb onto the heterogeneous cathode catalyst layer rather than dissociating to form two N adatoms as in the Haber-Bosch process (Fig. 1.5). The adsorbed N2 undergoes multiple proton and electron transfer steps to eventually form NH3 which desorbs from the catalyst layer and is released to the surrounding electrolyte solution. 19 The exact sequence for the transfer of H onto the adsorbed N2 depends on the electrocatalytic surface, which can be theoretically predicted by different electrochemical models in the framework of DFT. In-situ characterization techniques to detect surface-adsorbed species during the ENRR can provide additional evidence in corroboration with theoretically modelled reaction pathways. Commonly proposed hydrogenation pathways are depicted in Fig. 1.5, which include: (1) alternating (two N atoms are reduced alternatively); (2) distal (only one N atom is reduced consecutively to release one NH3 prior to the hydrogenation of the other N); (3) Mars-Van Krevelen (N-containing surface is reduced to form NH3 which leaves a N-vacancy to be Fig. 1.4 Basic cell layouts for the NH3 electrosynthesis are classified based on the presence of a separator between electrodes and the reactant phases. A – D: non-separated (NS), liquid-liquid (L-L), gas-liquid (G-L), and gas-gas (G-G) separated cells, respectively. E: the schematic illustration of a membrane-electrode assembly (MEA) that can be incorporated in separated cells for improved mass transfer of reactants. The catalyst layer (CL) can attach to the gas-diffusion layer (GDL) or the membrane separator, resulting in a gas-diffusion electrode (GDE) or catalyst-coated membrane (CCM) architecture, respectively. F: as a combination of L-L and G-L cell configurations, a gas-liquid-liquid (G-L-L) three-compartment cell has been used in some ENRR studies to expose the CL of the cathode GDE to both the gaseous and liquid phases.38,39,53 20 replenished by newly adsorbed N2); and (4) surface hydrogenation (H adatom is firstly formed on vacant active sites to initiate N2 adsorption with the direct formation of N2H2). Meanwhile, a platinum anode is commonly used (sometimes graphite) and expected to perform the oxygen evolution reaction (OER, Equation 1.9). A reference electrode (e.g., silver/silver chloride electrodes) is used to record the working electrode potential. However, NH3 products from the ENRR can dissolve in the solution or become mostly protonated as ammonium ions Fig. 1.5 Different reaction pathways for the adsorption, activation, and hydrogenation of N2 to form NH3. A: the dissociative N2 adsorption occurs in the Haber-Bosch where the high temperature and pressure drive the N≡N cleavage on a catalytic surface, which is subsequently combined with H adatoms to form NH3. B – E: at ambient conditions, N2 molecules are adsorbed by the electrochemical sites associatively. The N≡N bonds are not cleaved upon the chemisorption but are gradually elongated by multiple steps of proton-coupled electron transfer. Distal, alternating, Mars-van Krevelen, and surface hydrogenation pathways are named after the locations where the initial hydrogenation process occurs, which are: single N atom of the chemisorbed N2, both N atoms of the chemisorbed N2, surface N of a metal nitride, and electrocatalyst surface, respectively. 21 (NH4+) at a pH above 9.3. As a separator is absent from NS cells, ENRR products (NH3 or NH4+) can reach the anode by diffusion or convection with minimal restriction, where they are subject to oxidative decomposition with an underestimated ENRR performance.46,47 In previous studies, the NS cell configuration is not commonly used (Fig. 1.3). Nevertheless, with a low cost and a straightforward setup, NS cells remain useful for the preliminary evaluation of electrocatalyst candidates for the ENRR. On the other hand, installing a separator between the working and counter electrodes offers a simple solution to reduce product crossover (Fig. 1.4B). Therefore, the L-L separated cell (or so-called “H-type” cell) is the most popular choice for NH3 electrosynthesis applications. In both NS and L-L cells, N2 gas needs to be firstly dissolved from the gas supply (gas bubbles) to the liquid phase and then carried to the electrode-electrolyte interface for the N2 reduction. For N2 molecules to reach the ENRR active site, the liquid electrolyte solution and the electrode-electrolyte interface need to be considered to improve the N2 solubility and to reduce the mass transfer resistance in the bulk and near the catalyst layer surface, which will be covered in later sections. Besides acting as a physical barrier to prevent product crossover, using a separator (e.g., ion-conductive, or porous membranes) can also serve as a solid electrolyte for the ion conduction, sometimes in place of the liquid electrolyte. That is, when the catalyst layer is deposited or pressed onto the separator with intimate contact, proton can be delivered via the solid electrolyte for N2 4H+(aq.) + 4e– + O2(g) ↔ 2H2O(l) E°298 K = 1.229 VSHE at pH = 0 (1.9a) 2H2O(l) + 4e– + O2(g) ↔ 4OH–(aq.) E°298 K = 0.401 VSHE at pH = 14 (1.9b) H2(g) ↔ 2H+(aq.) + 2e– E°298 K = 0 VSHE (1.10) 22 reduction, resulting in a membrane electrode assembly (MEA) configuration (Fig. 1.4E). Such design, therefore, allows gaseous reactant(s) to enter the electrolytic cell and reach the catalyst layer directly, which results in the gas-liquid (G-L) cell layout (Fig. 1.4C). Multiple benefits can be derived from such a cell design: (1) improved N2 mass transfer since the gas-liquid (N2-liquid electrolyte) interface is absent; (2) reduced HER activity since the concentration of proton donor (e.g., H2O or H3O+) is reduced as opposed to abundant amounts in liquid-filled cells; (3) reduced cell resistance as the electron-insulating property of the separator allows for the close positioning of the cathode and anode without a gap.26,38,48,49 In addition to having gaseous N2 in the cathode compartment, H2 gas can also be fed to the anode compartment in a gas-gas separated cell (Fig. 1.4D) and reacts in the hydrogen oxidation reaction (HOR).48,50–52 Since the HOR (Equation 1.10) is kinetically facile, it can act as the pseudo-reference in G-G cells to allow for a two-electrode setup without the necessary use of the reference electrode, and the measured cell voltage can then be conveniently converted to the ENRR potential versus the reversible hydrogen electrode (RHE). Moreover, considering the more favourable thermodynamics of HOR over the OER, employing a gas-filled electrolyzer often improves EE of NH3 electrosynthesis.49 Similarly, by feeding H2 to the anode, the fast kinetics of the HOR can inhibit the undesired NH3 oxidation if the product crossover occurs. Nevertheless, the feasibility of such design needs to consider the energy and environmental cost to produce H2 feedstock (e.g., water electrolysis or water gas reforming) to meaningfully contrast with the incumbent Haber-Bosch process. Notwithstanding, it is still worth considering introducing gaseous N2 in the ENRR cell design and fully taking advantage of the triple-phase interface in managing the microenvironment surrounding the ENRR active sites, especially regarding the regulation of local pH and N2 access. Innovative 23 designs beyond the four common cell layouts have been proposed. For example, as a hybrid of the L-L and G-L cells, a gas-liquid-liquid (G-L-L) cell can be formed by adding another liquid-phase compartment between the ENRR cathode and the separator (Fig. 1.4F). With this cell design, the composition and flow conditions of the gaseous and liquid reactants on the cathode side (i.e., N2 and proton donor) can be independently manipulated in the respective gas- and liquid-phase chambers. As a result, one can maximize the triple-phase interface by precisely regulating the exposure of the electrode catalyst layer to the gas and liquid phases. With such a benefit, G-L-L cells have found applications in recent ENRR studies, especially in Li-mediated non-aqueous systems where N2 mass transfer can become the rate-limiting factor rather than its reactivity.38,39,53 1.4 Cell Component Design As commonly shared features, the choice of separator, electrolyte composition (solute and solvent), and cathode structure can have a direct or indirect influence on the cell interfaces. In this section, these components will be discussed to exemplify cell design strategies proposed to date. 1.4.1 Separator As mentioned before, separators in NH3 electrosynthesis cells are crucial in preventing product losses and completing the charge transfer processes by conducting ionic species other than NH3/NH4+. In gas-phase reactors, ion-conducting membrane separators can serve as a solid electrolyte in place of liquid electrolyte solutions to transfer proton donors (H2O or H3O+) for the ENRR. Hence, low product permeability and satisfactory proton conductivity should be two general design objectives in selecting appropriate separators for NH3 electrosynthesis. The most common material to separate the cathode and anode is the cation-exchange membrane (CEM), a group of polymeric materials that can selectively transport cation ions. Nafion, the commercial brand made of a polytetrafluoroethylene (PTFE) backbone and perfluoropolyether 24 side chains terminated by sulfonic groups (–SO3H) groups. Once fully hydrated, the pendant sulfonic acid groups in the Nafion membrane are clustered together and form micropores (inverted micelles) of ~ 4 nm (40 Å), which are interconnected by channels of ~ 1 nm (10 Å).54 The pore’s interior is highly hydrophilic and attracts H2O molecules closely so that protons can be percolated through the surface-bound H2O. Meanwhile, protons can be transported within the channel made of sulfonic groups following a proton-hopping mechanism (or so-called Grotthuss mechanism). The surface percolation and proton-hopping mechanisms collectively contribute to a high proton conductivity up to 0.2 S cm–1 in the Nafion membrane, thus making it the most popular choice of separator in NH3 electrosynthesis studies. However, the principal concern regarding CEMs including Nafion is the accumulation of NH4+ within the membrane, resulting in membrane degradation and cell contamination. During NH3 electrosynthesis, the Nafion membrane was observed to absorb NH4+ product from the cathode compartment.27,48,51,55 As NH4+ permeates into the membrane, the separator may lose its ability to obstruct the product movement, resulting in the discharge of Nafion-absorbed NH4+ into the anode compartment.27,46,51,56 For instance, Ren et al.46 observed NH4+ crossover within two hours with three common Nafion products (115, 117, and 211) used as the separator. The product loss became more significant when a more negative potential was applied (Fig. 1.6A – B). The attachment of NH4+ within the Nafion membrane was also reported and correlated with the overall thickness of the separator.27 25 As another possible drawback, an increased amount of NH4+ within the Nafion membrane may also degrade the structural integrity of sulfonic groups, consequently compromising the membrane conductivity and durability in NH3 electrosynthesis.46,57 As an alternative, Lan et al.58 modified the Nafion 211 membrane by soaking it in ammonia solutions to acquire NH4+/H+ co-conducting Fig. 1.6 Choosing the separator material for the NH3 electrosynthesis needs to consider its affinity to NH3/NH4+ products and proton conductivity. A: the NH3 concentration decreases and increases over time in the cathode and anode compartments, respectively, as separated by Nafion membranes. B: the NH3 concentration remains constant at open circuit potentials in a NS cell but decreases with applied cathode potentials over time. Reproduced from Ren et al.46 with permission. Copyright 2019, John Wiley and Sons. C: the NH3 adsorption and release by microporous diaphragms (Celgard) and CEMs (Nafion) over a 2-hr period. 0.1 M KOH with and without 0.5 ppm NH4+ (15 µg in total mass) were used for the adsorption and release process, respectively. Reproduced from Liu et al.27 with permission. Copyright 2020, Elsevier B.V. D: increasing the total pore area of a Celgard 3401 membrane improves the rNH₃,geo initially and then plateaus, whereas a decreasing trend was observed for the FE. Reproduced from Liu et al.69 with permission. Copyright 2018, American Chemical Society. 26 capability for a more consistent operation. However, the accumulated NH4+ in Nafion can become a source of false positives with repeated usage from consecutive experiments.31 Therefore, researchers should be aware of these shortcomings using CEMs particularly the Nafion membrane, and employ proper cleaning procedures. For example, Hanifpour et al.44 proposed a series of pre-treatment steps to remove NH4+ from the Nafion membrane including sonicating the membrane briefly in deionized H2O and 0.05 M H2SO4 solutions at room temperature immediately before NH3 electrosynthesis. Alternatively, the anion-exchange membrane (AEM) can be used as the separator to conduct hydroxyl ions (OH–) that are formed in alkaline ENRR. The AEM is usually structured with an aliphatic or aromatic polymer backbone, such as poly(arylene ether), polyphenylene, polypropylene, poly(arylene ether ketone) or polyfluorene, etc.; and functionalized with quaternary ammonium groups (NR4+).59–63 Despite rapid development in recent years, the AEM still has issues in stability, inherently slow OH– mobility, and susceptibility to structural degradation to leach the functional NR4+ groups in high alkalinity.64 The leaching process in particular not only can contribute erroneous non-catalytic NH3 to the electrolytic system but also lead to operation instability, therefore warranting the researcher’s awareness and control experiments to exclude false positives. Microporous diaphragms are another group of polymeric thin-film material used in NH3 electrosynthesis due to their inertness to NH4+ uptake and chemical and thermal stability. So far, research on microporous polymer diaphragm (typically polyethylene and polypropylene) is being led by the lithium-ion battery development, where the membrane’s porosity, pore-size-distribution, and tortuosity can be correlated with the battery performance including the Li+ flux and dendrite mitigation.65 For NH3 electrosynthesis, the microporous polypropylene diaphragm is 27 considered as an effective non-ionic barrier to restrict the movement of NH4+ due to its excellent chemical and thermal stability.27,66 However, the ion conductivity in microporous diaphragms mostly originates from the diffusion process across the micropores (normally below 0.3 μm in diameter) when filled with electrolyte solutions. Because of its polymeric hydrocarbon chain, the polypropylene diaphragm surface is usually hydrophobic and poorly wettable in aqueous ENRR systems, unless coated with hydrophilic materials (e.g., polyacrylonitrile).67,68 With insufficient electrolyte uptake, the ionic conductivity of a microporous diaphragms is reduced, resulting in high cell resistance and energy consumption in aqueous systems. In this regard, non-aqueous electrolyte systems are more compatible with microporous diaphragm separators to ensure both product confinement and sufficient ion conductivity. For example, Liu et al.27 recommended using polypropylene diaphragm (Celgard) over Nafion membrane due to lower NH4+ absorption. In 30 mL of 0.1 M KOH with 0.5 ppm NH4+, Celgard diaphragm absorbs about 0.2 µg NH4+ over 2 hr which is far less than 1.62 µg NH4+ absorbed by Nafion 115 membrane (Fig. 1.6C). However, they observed significant cell resistance when constructing the L-L cell with Celgard 2400 due to its hydrophobic surface. By contrast, Celgard 3501, a surfactant-coated alternative, has better wettability in an aqueous solution, which lowers the open cell voltage significantly. Furthermore, Liu et al.69 proposed a pore-size control strategy to tune the proton flux across the Celgard diaphragm in an L-L cell. By puncturing 0.1-mm macropores in addition to the diaphragm’s original micropores, the overall pore area was increased to permit additional proton transfer in a neutral electrolyte (0.1 M Na2SO4), increasing the rNH₃,geo to a fair amount of 2.0 x 10–10 mol s–1 cmgeo–2. However, the FE is lower with larger pore openings, decreasing from 41.9% to less than 20% for an Ag-Fe catalyst (Fig. 1.6D). Therefore, how to 28 properly select or rationally modify the type, composition, and structure of microporous polymer diaphragm implies great potential in its use for NH3 electrosynthesis. Lastly, poly(benzimidazole)-derived CEMs and AEMs are potential separator candidates for NH3 electrosynthesis particularly in G-G cells, given their thermally and chemically stable backbones and resistance to dehydration at mild temperatures. Glass frit/fibre separators have also appeared in ENRR studies but the removal of NH4+ from the glass perforations can be difficult.41,70–72 Concentrated salt bridges were suggested as well but their practicability needs experimental confirmation.46 1.4.2 Liquid Electrolyte In recent years, the composition of the liquid environment has drawn increasing attention in the field of NH3 electrosynthesis. In liquid-phase cells, the electrolyte solution provides ionic conductivity to lower the cell ohmic resistance and dissolves N2 and proton donors whose concentrations are proven critical in obtaining good ENRR performance. Key considerations for electrolyte design include the identity and concentration of the solute species and the choice of solvent, which relate closely with the relative amounts of N2 and proton, and eventually the competition between the desired ENRR and unwanted HER. Lastly, the appearance of non-aqueous electrolytes in Li-mediated applications will be briefly discussed. 1.4.2.1 Solute Species in Liquid Electrolyte For the cation species, lithium (Li+) and potassium (K+) ions have been reported as promotors in previous ENRR studies, as they can suppress the HER while improving the ENRR activity.38,73–78 Interestingly, for the Haber-Bosch process, the promoting effect of atomically adsorbed alkali metal species was also observed, which can be correlated with the electronic charge transfer from the promotor (e.g., K) to active sites (e.g., Fe) to enhance N2 adsorption.79,80 However, for NH3 29 electrosynthesis, mechanistic understanding of the promoting effect from alkali metal ions remains to be described in detail, especially on supporting which alkali metal ion works optimally. So far, the intercalation of Li+ and the formation of the K+ solvation shell on catalyst layer have been proposed as two possible reasons. As an example, Li+ ions were incorporated into a poly(N-ethyl-benzene-1,2,4,5-tetracarboxylic diimide) catalyst via cyclic voltammetry in 0.5 M Li2SO4, which depresses the HER onset potential in H2SO4 from –0.22 VRHE to –0.61 VRHE.76 The Tafel slope for the HER was measured as 969 and 356 mV dec–1 in 0.5 M Li2SO4 and 0.5 M H2SO4 solutions, respectively, indicating a kinetically hampered HER activity. Although only a low FE of 2.85% and an rNH₃,geo of 0.30 x 10–10 mol s–1 cmgeo–2 was recorded, the HER-suppressing role of Li+ remains inspiring. From in-situ X-ray diffraction (XRD) and ex-situ X-ray photoelectron spectroscopy (XPS), Patil et al.81 observed the intercalation of Li+ from LiClO3 aqueous solution to 1T-phase MoS2 during the ENRR, which improves N2 adsorption while destabilizing H attachment on the catalyst layer, resulting in a good rNH₃,geo of 10.3 x 10–10 mol s–1 cmgeo–2 with 27.66% FE (Fig. 1.7A – Fig. 1.7C). Fig. 1.7 The effect of Li+ incorporation to the ENRR performance. A: the Li-1s XPS spectra supports the intercalation of Li into 1T-MoS2. B: the in-situ XRD patterns for the 1T-MoS2-loaded electrode collected during linear scan voltammetry from 0 to −0.6 VRHE in the N2-saturated LiClO4 solution. The peak shift observed from 9.5° to 7° with more negative potentials indicates the intercalation of Li into the 1T-MoS2. C: the rNH₃,geo and FE of the ENRR from 1T-MoS2 obtained in 0.25 M LiClO4 solution. Reproduced from Patil et al.81 with permission. Copyright 2020, Royal Society of Chemistry. 30 In comparison, the intercalation of K+ to an ENRR electrocatalyst is generally more difficult, largely owing to a larger ionic radius. Therefore, structurally integrating K+ to promote the ENRR was mostly done during the preparation of electrocatalysts, resulting in electronic modification of active sites for more facile adsorption or activation of N2.77,78 On the contrary, the contribution of K+ ion to the ENRR as an electrolyte solute most likely originates from the solvated shell made of H2O. Solvated K+ ions can concentrate at the electrochemical double layer under potential and have shown better ENRR enhancement than other monovalent metal ions (Li+, Na+, Rb+, and Cs+) for bismuth and copper-based electrocatalysts.38,73,74 This superior improvement by K+ was credited to the optimal ion radius under solvation to block H adsorption (Fig. 1.8A). Notably, the excellent ENRR performance (67% in FE and 16.6 x 10–10 mol s–1 cmgeo–2 in rNH₃,geo) from bismuth nanocrystals promoted by concentrated K+ ions (1 M) were among the highest reported values so far,73 despite questions on lacking NMR verification and reproducibility.82 For the anionic species, no promoter has been conclusively demonstrated to enhance the ENRR in aqueous solutions. Phosphate (PO43–) ions were reported to beneficially increase the N2 solubility Fig. 1.8 The effects of different types and concentrations of ionic species in the aqueous electrolyte. A: the rNH₃,geo and FE obtained at −0.5 VRHE from a Cu-based electrode as a function of different cations. The same concentration (0.32 M) was used in all electrolyte solutions. Reproduced from Kani et al.38 with permission. Copyright 2020, American Chemical Society. B: the ENRR performance obtained in 1 M KOH, KCl, and KSCN electrolyte solutions in both potentiostatic (−0.6 VRHE) and galvanostatic (−10 mA cm−2) operations. Reproduced from Li et al.83 with permission. Copyright 2021, John Wiley and Sons. C: the NH3 yield rate as a function of the cathode potential and the LiCl concentration. Reproduced from Wang et al.85 with permission. Copyright 2021, Nature. 31 and synergistically promote the ENRR with K+.74 However, the pH-buffering ability of PO43– by forming conjugated acids (HPO42– and H2PO4–) was not fully examined. In a recent study, conducting NH3 electrosynthesis in 1 M KOH was observed to achieve higher performance than in 1 M KSCN and KCl for both potentiostatic and galvanostatic operations (Fig. 1.8B).83 However, when adding 0.01 M OH−, Cl−, or SCN− anions to 0.1 M phosphate buffer solution (pH = 7), the authors found that varying the anion selection independent of pH changes only improves the ENRR performance negligibly, inferring that the proton donor concentration plays a more critical role in improving alkaline NH3 electrosynthesis. Moreover, NO2– and NO3– anions should be removed from aqueous electrolyte solutions to eliminate questionable results since their reduction to NH3 can proceed at much less negative equilibrium potentials.33,36,82 Therefore, before and after the electrolysis, one should consider performing ultraviolet spectrophotometry and Griess diazotization colorimetry for the respective detection of NO3– and NO2– in the electrolyte solution to ensure they are not introduced prior to or during NH3 electrosynthesis.84 Additionally, the solute concentration matters in controlling the availability of protons in aqueous solutions due to ion solvation. Wang et al.85 theorized and experimentally confirmed that 10 M LiCl aqueous solution has the optimal solute concentration to disrupt the H2O-H2O interaction while solidifying the H2O-ion interaction, therefore contributing to a significant reduction in free H2O molecules and lower HER activities (Fig. 1.8C). Apart from being an HER inhibitor, the strong Li+ solvation also interrupts the weak H2O-N2 interaction, which forces the dissolved N2 to transfer out of the liquid phase and preferentially coalesce onto the electrode surface. Owing to the solvation and salting-out effects in the concentrated solution, the boron-decorated carbon catalyst obtained an excellent FE of 71% and noticeable rNH₃,geo of 9.5 x 10–10 mol s–1 cmgeo–2 at –0.3 VRHE. Similarly, Guha et al.86 also demonstrated that highly concentrated Li+ (5 M) in aqueous solutions 32 can effectively promote the ENRR by stabilizing the surface adsorbed N on polycrystalline metals (Cu and Au). Of note, trace amounts of NO2– or NO3– were found in impure electrolyte salts (e.g., Li2SO4) and may be enriched as the solute concentration increases, which is a concern for reliable results. Recently, ionic liquids (ILs)—a group of aprotic, electrochemically stable, and ionically conductive liquid materials have found applications in Li+ ion batteries, solar cells, and fuel cells.87 As for NH3 electrosynthesis, ILs are used primarily for their high N2 solubility in comparison with aqueous conditions. For instance, a fluorinated IL ([P6,6,6,14][C8F17CO2]) was found to dissolve up to 6.5 mM N2 at 30 °C and 1 atm, whereas only 0.6 mM N2 can be dissolved in H2O at ambient conditions.88 Additionally, ILs have controllable levels of hydrophobicity depending on their anionic and cationic constituents, which makes it possible to use ILs to restrict the proton access at the electrode surface. Hence, the availability of both N2 and proton can be adjusted intentionally by incorporating IL in an aqueous or non-aqueous system, resulting in better selectivity toward the ENRR.41,45,89 For instance, Du et al.45 reported an excellent FE of 85% from Mo@MoS2 using an IL-based electrolyte ([C4mpyr][eFAP]) containing 6 mM H2O as the proton donor (Fig. 1.9A – B). However, the apparent performance degraded after 20 min, resulting in a low rNH₃,geo of 0.002 x 10–10 mol s–1 cmgeo–2 for a 12-hr average, which indicates a minimal level of proton supply must be jointly provided. Apart from ionic species, molecular additives that can modify the electrode surface are discovered to retard the HER kinetics for better ENRR selectivity. For example, Guo et al.90 mixed 20 wt% polyethylene glycol to the aqueous 0.05 M H2SO4 or 0.1 M NaOH solution and observed enhanced rNH₃,geo and FE for both acidic and alkaline conditions (Fig. 1.9C). The suppressed HER was 33 attributed to the reduction in proton donor activities with the formation of numerous hydrogen bonds between macromolecules and H2O or H3O+ (or the so-called molecular crowding effect). 1.4.2.2 Solvent Type and Property in Liquid Electrolyte Depending on the dielectric constant and dipole moment (Table 1.2), different solvents thermodynamically have various tendencies to donate protons that participate in both the ENRR and the HER. Altering the liquid electrolyte from aqueous to less protic or aprotic ones can be a strategy of controlling the proton flux to boost the ENRR selectivity.91–93 Focusing on controlling the proton donor accessibility, it is important to discuss the effects of pH in aqueous electrolyte solutions and provide examples where non-aqueous solvents are successfully applied. In neutral and alkaline aqueous solutions, the majority of H2O exists in its molecular form, whereas in acidic conditions, most H2O is protonated to H3O+ ions. The kinetics for H2O to participate in the alkaline HER has been computationally and experimentally observed to be more sluggish than H3O+ in acids. The observed pH-dependence of the HER has been largely attributed to the higher Fig. 1.9 ENRR experiments conducted in non-aqueous electrolyte. A: the effect of trace H2O concentration in the ionic liquid on the overall NH3 yield and the observed performance degradation in the ENRR. B: the observed FE toward the ENRR is also affected by the H2O content in the IL and the electrolysis duration. Reproduced from Du et al.45 with permission. Copyright 2020, The Electrochemical Society. C: The ENRR performance can be enhanced by adding 20 wt% polyethylene glycol to either 0.05 H2SO4 or 0.1 M NaOH solutions. Reproduced from Guo et al.90 with permission. Copyright 2021, Nature. 34 activation energy required to dissociate H2O molecules to form adsorbed hydrogen (i.e., the Volmer step).93–96 Table 1.2 Physical Properties of Aqueous and Non-aqueous Solvents for NH3 Electrosynthesis. a Data for solvents at 25 °C, unless specified otherwise. b Data for solvents at 1.013 bar partial pressure of N2. c Data taken from 113 unless specified otherwise. As for the ENRR, the pH variation has not been conclusively proven to improve or deteriorate the overall performance. NH3 electrosynthesis conducted in alkaline or neutral conditions seems to be favoured in achieving a higher FE, but the decrease in proton availability may reduce the corresponding rNH₃,geo. For example, Mukherjee et al.114 compared the ENRR performance of a nickel-based catalyst in KOH, LiClO4, and HCl solutions (pH = 13, 8, and 1, respectively) and observed the highest FE from KOH, but the maximal rNH₃,geo was obtained from the LiClO4 instead (Fig. 1.10A – C). Interestingly, even within a small range (pH = 13 – 14), Kani et al.38 observed drastic changes in rNH₃,geo and FE (Fig. 1.10D), suggesting fine-tuning on proton concentration is needed to optimize the electrolyte design for NH3 electrosynthesis. Solvent Solubility of N2 in mole fraction (x 10–4)a,b Dipole moment (Debye)a,c Dielectric constanta,c Literature example Water 0.11897 3.12 77.5 Most studies Methanol 2.7397 2.97 32.7 70 Ethanol 3.5797 1.66 24.6 40 2-propanol 4.6697 3.09 19.9 98 Ethylenediamine N/A 2.2299 15.1 (27 °C)100 101 Tetrahydrofuran 4.97102 1.63 7.58 31,36,107–111,39,40,66,70,103–106 Acetonitrile 4.8112 3.39 37.5 84 35 In a recent study, the diffusion of dissolved N2 toward the electrode-electrolyte interface in an acidic environment (0.1 M HCl) was modelled focusing on the electric double layer (EDL) region.115 As illustrated in Fig. 1.10E, under applied potentials, H3O+ cations electrostatically migrate towards the cathode EDL and are densely packed within the outer Helmholtz plane (OHP), which forms a barrier to N2 diffusion and diminishes the ENRR performance as the applied potential becomes more negative. According to microkinetic data from Au-based catalysts, diluting the proton concentration is predicted to positively influence the ENRR activity and efficiency (Fig. 1.10F); however, when the proton concentration is overly low (pH > 3), the hydrogenation of N2 becomes rate-limiting for the ENRR whereas the N2 mass transfer is considered less critical. In another modelling work by Dražević and Skúlason,116 the onset potentials of the ENRR and the HER were both found to depend on the pH in aqueous solutions for a series of transition and post-transition metals. At pH = 0, the selected metals all require a higher onset potential for the ENRR than the HER, whereas at pH > 4, some metals (Mn, Ga and In) can initiate the ENRR at a lower potential than the HER. Moreover, the thermodynamic stability of metal materials with possible ENRR activities should be evaluated for applications at different pH. Some transition metals may form passivating oxide layers at high alkalinity, possibly losing the intrinsic ENRR activity altogether. Hao et al.117 found that Pt-alloyed WO3 nanoplates as the ENRR active species were corroded in alkaline (0.1 M KOH) instead of neutral (0.1 M K2SO4) or acidic electrolytes (0.05 M H2SO4), resulting in the lowest FE among the three electrolyte variations. 36 Fig. 1.10 Optimizing the pH level in aqueous systems. A – C: performances from atomically dispersed Ni with different pH (13,1, and 8 respectively). Reproduced from Mukherjee et al.114 with permission. Copyright 2020, John Wiley and Sons. D: the subtle difference in pH of an alkaline electrolyte (KOH) was found to influence the ENRR performance. Reproduced from Kani et al.38 with permission. Copyright 2020, American Chemical Society. E: the schematic of the electrode-electrolyte interface including the electric double layer (EDL) in N2-saturated HCl solution. When applying a negative potential, H+ are electrostatically attached to the cathode, especially within the outer-Helmholtz plane (OHP), which blocks the access of dissolved N2 to the active sites. F: the ENRR performance of a gold catalyst as a function of pH. Reproduced from Tao et al.115 with permission. Copyright 2021, American Institute of Chemical Engineers. 37 It is well known that the solution pH in the bulk solution or at the electrode surface can change temporally in long-term or high-current-density electrolysis.48,118 The consumption of proton from cathode reactions increases the local pH whereas the anodic oxidation of H2O or OH– acts the opposite, therefore leading to heterogeneity in alkalinity at the electrode-electrolyte interface and added polarization in cell voltage.119,120 Buffering solutions such as the phosphate buffer solution have been used in ENRR studies with satisfactory results, likely owing to their capability of hampering the pH fluctuation,74,121,122 but their application has not been widely advocated by theoretical or experimental evidence. Instead of H2O, both protic and aprotic solvents have been used in NH3 electrosynthesis to create a proton-restrained environment. Protic solvents (e.g., methanol and ethanol) are usually mixed in a small proportion (roughly 1 wt%) with an aprotic bulk solvent (e.g., tetrahydrofuran). In such systems, the protic solvent and trace H2O is considered as the proton donor to provide the need for N2 hydrogenation while reducing the H2 formation. For instance, Lee et al.40 prepared 0.2 M LiCF3SO3 in a blend of ethanol and tetrahydrofuran solvent to obtain a fair FE of 18%. However, the NH3 production rate was at an unsatisfactory level of 0.10 x 10–10 mol s–1 cmgeo–2, likely because the reduction in proton supply can decrease the HER as well as the desired ENRR activity simultaneously. In addition to adjusting the proton donor concentration, increasing the dissolved N2 concentration to enhance the ENRR performance is another objective that can be achieved with non-aqueous electrolyte systems. Compared with H2O, N2 gas is more soluble in hydrocarbons and organic solvents containing oxygen, nitrogen, or halogen.97 Fluorinated aprotic solvents, in particular, have been used in conjunction with fluorinated ILs to collaboratively increase the availability of N2 at the electrode-electrolyte interface.123 For example, Suryanto et al.89 tested iron nanorod catalysts 38 in a homogeneous mixture of an IL ([C4mpyr][eFAP]) and a fluorinated solvent (FPEE) as the electrolyte, which also contains trace amounts of moisture as the proton source. The optimal mole fraction for [C4mpyr][eFAP] was determined to be 0.23 to balance several factors including the viscosity, conductivity, N2 solubility and diffusivity of the solution (Fig. 1.11A – B). With 114 ppm H2O in the non-aqueous system, a good FE of 23.8% was obtained, despite a low yield rate of 0.158 x 10–10 mol s–1 cmgeo–2 ((Fig. 1.11C – D)). Both the FE and rNH₃,geo can be improved to 30.8% and 0.235 x 10–10 mol s–1 cmgeo–2, respectively, simply by adding a higher quantity of H2O Fig. 1.11 Use of non-aqueous solvents for the ENRR. A – B: the N2 solubility and ionic conductivity in solutions containing an ionic liquid ([C4mpyr][eFAP]) and a fluorinated aprotic solvent (FPEE). Reproduced from Kang et al.123 with permission. Copyright 2019, American Chemical Society. C – D: the ENRR performance of an iron-based electrode in a non-aqueous electrolyte at various applied potentials and solute-solvent ratios. Reproduced from Suryanto et al.89 with permission. Copyright 2018, American Chemical Society. 39 to the non-aqueous electrolyte (144 ppm). Therefore, both the dissolved N2 and the residual moisture in the non-aqueous electrolyte should be carefully monitored and controlled to balance the activity and efficiency aspects of the ENRR. N-containing solvents (e.g., ethylenediamine), however, should be avoided in non-aqueous systems, as NH3 might derive from these chemicals, leading to inaccurate assessment in NH3 electrosynthesis.101 Moreover, like aqueous systems, conducting long-term electrolysis in non-aqueous electrolyte solutions can also lead to local pH increases. Recently, Hodgetts et al.84 detected NOx impurities from the N2 gas supply which were believed to be strongly adsorbed by the Fe-based electrode and only released to the electrolyte as the local pH changes. This phenomenon likely contributed to the persistent increase of NO2– and NO3– from zero to a total of nearly 10 µM in an acetonitrile-based non-aqueous electrolyte solution. Therefore, continuously monitoring adventitious nitrogen sources in non-aqueous ENRR experiments is encouraged to avoid false positives with causes different from those observed in aqueous systems. 40 1.4.2.3 Li-Mediated Non-Aqueous Systems As a unique approach to achieving the N2-to-NH3 conversion, Li-mediated non-aqueous electrolysis has drawn increased interest in the field of NH3 electrosynthesis in order to circumvent the activity and selectivity challenges in aqueous conditions. As early as 1993, Tsuneto et al.110 reported the presence of NH3 products from a non-separated cell that contains LiClO4 (0.2 M) dissolved a mixture of tetrahydrofuran and ethanol (99:1 in volume) as the electrolyte. Among different metal wires that were used as the cathode, Ag achieved the maximum NH3 yield of 14.5 µmol with an 8.4% FE after passing a total charge of 50 °C, which can be further improved by pressurizing the cell up to 50 atm. The presence of Li+ ions from lithium salts (LiClO4, LiBF4, and Li(CF3SO3)) was attributed as the key in reducing N2, as negligible NH3 was detected from systems that contain Na+ or tetrabutylammonium (Bu4N+) cations.110,111 Consequently, the electro-deposition of Li metal from Li+ to the electrode surface was inferred as the first reaction step (Fig. 1.12A). Then, with a large negative Gibbs free energy change (∆G°298 K) of –135.4 kJ molLi3N–1, Li metal can easily chemically react with N2 to form lithium nitride (Li3N) which is quickly hydrolyzed with the proton donor (e.g., ethanol) to form NH3 and Li+. Interestingly, both the type and concentration of the proton donor can affect the NH3 yield and efficiency of the proposed Li-mediated system (Fig. 1.12B – C). This early study paves the road to build and operate Li-containing nonaqueous systems for the ENRR, and variation of cell design parameters also appears critical in recent publications.66,124,125 41 Fig. 1.12 Mechanistic understanding and evidence for Li-mediated non-aqueous NH3 electrosynthesis. A – B: the ENRR performance with distinct types of H+ donors and optimization with EtOH, respectively. MeOH, EtOH, n-PrOH, i-PrOH, and i-BuOH represent methanol, ethanol, n-propanol, isopropanol, and 2-methyl-2-propanol, respectively. Reproduced from Tsuneto et al.126 with permission. Copyright 1994, Elsevier B.V. C: the charge density distribution in forming NH3, Li, and Li3N over the 4-hr electrolysis, which was considered as evidence for these species being stable catalytic sites in the ENRR. Reproduced from McEnaney et al.126 with permission. Copyright 2017, Royal Society of Chemistry. D: the stepwise Li-mediated model that consists of ① Li electro-deposition, ② Li nitridation/N≡N cleavage, and ③ Li3N hydrolysis/Li regeneration. E: the heterogeneous electrocatalysis model that describes the electro-deposited Li and its derivatives (Li/LiH/Li3N) as active sites to facilitate the N2 adsorption and hydrogenation similarly as in aqueous ENRR conditions. 42 However, details regarding the ascribed stepwise process have not been fully elucidated, particularly on the composition and role of Li-related deposits.108,109 By anodic Li stripping and H2O hydrolysis, Schwalbe et al.109 calculated the charge distribution towards the formation of Li and Li3N, respectively, which was then compared with the charge consumed for the NH3 production (Fig. 1.12D). In contrast to the regenerative and sequential view of the presence of Li and Li3N, the authors proposed that the electro-plated Li and its nitride and hydride derivatives are stable surface deposits that can also act as active sites for the electrochemical N2 activation and hydrogenation (Fig. 1.12E). In particular, NH3 was produced continuously over the 4-hr electrolysis period at –1.1 mA cm–2, whereas a large quantity of charge was supplied to the Li deposition over the initial hour and soon reached steady-state. The surface Li3N was observed to have a similar trend as Li except for a relatively small overall charge density (0.05 C cm–2). The detection of Li and Li3N of a consistent amount in the system indicates that a certain equilibrium state can be reached after the initial electrolysis, resulting in a stable surface composed of Li, Li3N, and LiH, which can be supported by the thermodynamic and kinetic stabilities of these species. Additionally, the ENRR via the associative pathway was calculated to have limiting potentials of –0.76, –1.33, and –1.53 VRHE for Li, Li3N, and LiH, respectively, which are easily reachable under the operating potentials for the electroreduction of Li. Nevertheless, when a Li-deposited electrode is exposed sequentially to the N2 gas and proton donor, the reaction scheme illustrated in Fig. 1.12D remains as the most agreeable route. An early study by McEnaney et al.126 calculated the free energy changes during a batch-like operation that consists of three consecutive steps: Li electro-deposition, Li nitridation, and Li3N hydrolysis to form NH3 (Fig. 1.13A), which are all thermodynamically feasible with a sufficiently applied potential. As a proof of concept, they prepared a molten salt electrolyte (LiCl-KCl/LiOH-LiCl) to 43 accomplish the Li electro-deposition at 450 °C, which was then exposed to N2 after cooling down to room temperature, followed by hydrolysis in H2O. The diffraction pattern from ex-situ XRD Fig. 1.13 Understanding of the Li-mediated non-aqueous ENRR from a step-wise perspective. A: the phase diagram to demonstrate the cyclic process in the Li-mediated NH3 electrosynthesis. Reproduced from McEnaney et al.126 with permission. Copyright 2017, Royal Society of Chemistry. B: the in-situ XRD characterization for a stepwise Li-mediated electrolysis operation: ① electrolysis at –10.0 Vcell in Ar atmosphere, ② switching the gas supply to N2, and ③ adding 1% EtOH to the N2-saturated dry THF. Li (101) and Li3N (100) signals at 37.7° and 28.1°, respectively, were marked in the contour map for the Au-coated (a – f) and bare carbon paper (g – l). C: the ENRR performance of Au-coated and bare carbon paper (CP) electrodes Reproduced from Gao et al.103 with permission. Copyright 2021, John Wiley and Sons. 44 confirms the presence of Li metal and Li3N after the electrolysis and nitridation, respectively. Furthermore, due to a reasonably low electro-reduction potential and stable formation energy for nitrides, the authors also identified metals (Zr, Mg, V, Cr, and Ti) apart from Li as possible mediators to participate in such stepwise operation. Based on a similar batch-like operation, Gao et al.103 performed Li electro-deposition at ambient conditions with 0.2 M LiClO4 in dry tetrahydrofuran (THF) followed by consecutive nitridation with N2 and hydrolysis with ethanol (EtOH). The formation of Li and Li3N on a carbon paper substrate was evidenced by in-situ XRD during the sequence with their characteristic peaks being identified from the diffraction pattern (Fig. 1.13B – C). Moreover, the observed Li3N formation and consumption from the XRD spectra coincide with the time segments when the Li-deposited electrode was exposed to N2 and ethanol, respectively. At the current stage of development, enabling continuous and stable operation with low energy consumption is critical for the future scaleup to fully realize the potential of Li-mediated ENRR systems. To achieve this, the electrolysis must occur under the coexistence of Li salt, N2 supply, and a proton donor. However, many kinetic and thermodynamic aspects of such multicomponent systems are not fully elucidated, which is further complicated by practical issues such as Li under/over plating,104–106 electrolyte decomposition,104,127 electrode passivation with a solid-electrolyte interface (SEI),104,108,128,129 and insufficient mass transport of N2 or protons.39,66,107,130 Nevertheless, progress has been made in characterizing and optimizing the composition and interaction among the gas, liquid, and solid phases under continuous operation. More detailed introduction of cell design and operating strategies tailored for Li-mediated non-aqueous electrolysis will be covered in Chapter 5. 45 1.4.3 Cathode Design As discussed earlier, electrocatalyst development has been the main-stream design and research objective in the field of NH3 electrosynthesis, emphasizing active sites that can: (1) preferably chemisorb N2 over H, (2) initiate N2 hydrogenation with low activation energy, (3) desorb NH3 product quickly from the surface, and (4) distribute densely with uniformity and stability. Various strategies have been proposed including: (1) introducing O, N, or S vacancies, (2) doping or alloying metallic and/or non-metallic species, and (3) engineering structural defects to expose amorphous or low-coordination sites. However, summarizing the design approaches for the electrocatalyst composition and morphology is not the aim of this thesis. Instead, structural aspects of the electrode will be discussed with a focus on the catalyst layer architecture. 1.4.3.1 Catalyst Layer Surface Morphology Reducing the size or dimension of the catalyst material has been applied to prepare catalyst layers with a dense distribution of active sites, resulting in a larger electrochemical surface area (ECSA) available to adsorb and activate N2 molecules. Additionally, as the surface-to-volume ratio increases with smaller particle sizes, one can achieve higher utilization of active materials that are sometimes precise or complex in preparation. For example, Zhang et al.131 observed noticeable ENRR activities from boron nanosheets but negligible results from bulk boron powder (Fig. 1.14A – B), which was attributed to a larger ECSA and a lower charge-transfer resistance due to the size/dimension reduction. In addition, the catalyst morphology also contributes to the overall ENRR performance by enhancing the electric field and enriching the dissolved N2 concentration at the electrode-electrolyte interface. Song et al.72 found a 2V nm–1 enhancement in the local electric field at the tip of an N-doped carbon nanospike catalyst, which favours the electron injection to the 46 antibonding orbital of N2 and enables an rNH₃,geo of 15.9 x 10–10 mol s–1 cmgeo–2 with a fair FE of 11.6% at –1.2 VRHE. In comparison, the ENRR activity diminishes significantly when the nanospikes are removed by oxygen-etching (Fig. 1.14C). An aluminium-doped Co3O4 nanosphere catalyst with urchin-like nanospikes was synthesized and the sharp end of the nanospike was considered as the key trait to continuously remove H2 gas bubbles from the electrode surface for the improved N2 access to the active sites (Fig. 1.14D).132 Furthermore, employing porous electrocatalysts (or catalyst support) has shown the potential to increase the local N2 concentration within confined spaces. For example, Nazemi et al.133 Fig. 1.14 Surface modifications by size, shape, and porosity to synergically enhance the performance of ENRR. A – B: 2D B nanosheets in 0.1 M Na2SO4.in comparison with the bulk B powder. Reproduced from Zhang et al.131 with permission. Copyright 2019, American Chemical Society. C: carbon nanospikes outperform the controls due to locally enhanced electric fields. Reproduced from Song et al.72 with permission. Copyright 2018, American Association for the Advancement of Science. D: the urchin-like nanostructure on Al-Co3O4 nanospheres was shown to prevent the coalescence of gas bubbles for better N2 access to the active sites. Reprint from Lv et al.132 with permission. Copyright 2020, American Chemical Society. E: the hollow nanocage obtains the highest ENRR performance among different Au-based electrocatalysts with various shapes and geometries. Reproduced from Nazemi et al.133 with permission. Copyright 2018, Elsevier B.V. 47 synthesized gold nanoparticles with hollow cavities, which not only increases the ECSA of the catalyst layer due to a higher surface-to-volume ratio but also confined the dissolved N2 reactant within the hollow nanocages, leading to an excellent FE of 35.9%. However, the low rNH₃,geo of 0.6 x 10–10 mol s–1 cmgeo–2 remains to be improved. By contrast, Au nanorods, nanospheres, and solid nanocubes all achieved inferior NH3 production rates and efficiencies than the hollow nanocages at the same applied potential (Fig. 1.14E). Similarly, as a non-noble metal material, cobalt phosphide hollow nanocages were prepared using a metal-organic framework precursor and show a decent ENRR performance of 7.1 x 10–10 mol s–1 cmgeo–2 in rNH₃,geo at –0.4 VRHE and 7.36% in FE at 0 VRHE.134 As an alternative approach, porous catalyst supports can be prepared and incorporated with ENRR-active materials to synergistically improve N2 chemisorption and activation. For example, Lv et al.135 synthesized hollow TiO2 nanospheres as the support to load various metal dopants (Fe, Mn, Cu, Ni), which forms cost-effective electrodes with notable ENRR performances. The best result (7.1 x 10–10 mol s–1 cmgeo–2 in rNH₃,geo with 16.4% FE at –0.7 VRHE) was obtained from the Fe-doped hollow TiO2, which is nearly one order of magnitude higher than the Fe-doped solid counterpart. Of note, pore confinement to increase the local N2 concentration is valid in aqueous and non-aqueous systems. An IL 1-ethyl-3-methylimidazolium acetate (EmimOAc) confined in microporous carbon was recently discovered to dissolve more N2 than in bulk EmimOAc (51.9 and 4.3 cmN23 gIL–1, respectively), suggesting opportunities to incorporate a porous catalyst layer architecture with IL in non-aqueous NH3 electrosynthesis.136 Considering the wide array of morphologies that a nano-/micro-structured catalyst can adopt, it is nearly impossible to exhaust all possibilities within a relatively short time frame experimentally. Theoretical modelling and machine learning can provide a more holistic and efficient approach to optimizing electrocatalyst geometry. For example, Lu et al.137 numerically simulated the reaction 48 kinetics of the ENRR and HER using the finite element approach for catalysts with a planar or microwire geometry and identified that N2 supply can be severely depleted at the bottom of the microwire, shifting the reaction selectivity from NH3 to H2 formation. Based on a similar microkinetic model that also considers the blocking effect of the electrochemical double layer under applied potentials, Tao et al.115 compared the ENRR performance that can be obtained from a catalyst layer surface with a pore diameter of 10 nm and 50 nm. The N2 concentration within the pore tends to decrease with increasing depth, and such mass transfer limitation is more severe for ENRR electrocatalysts with higher exchange currents (Fig. 1.15A). Reducing the pore size can further limit the N2 diffusion from the bulk toward the microchannels. As a result, a higher ENRR current density is expected for the larger pore size, which is more pronounced when the active sites have a higher inherent activity (i.e., exchange current density) for the electron transfer to N2 (Fig. 1.15B – C). Furthermore, Hoar et al.138 integrated the microkinetic model with machine learning to accelerate the optimization of a microarray catalyst layer architecture. They systematically evaluated the importance of descriptors in surface morphology (e.g., wire length, diameter, and Fig. 1.15 A microkinetic model to study the effects of electrode dimension to the ENRR. A: the dissolved N2 concentration within nanopores decreases with depth, particularly for narrower pore channels (10 vs. 50 nm in diameter). The dissolved N2 becomes more depleted along the channel when the exchange current density for the electron transfer process (r0,1,NRR) increases from 10–5 to 10–3 mA cm–2. B – C: the current density for the ENRR is predicted to increase initially with more negative potentials before peaking at nearly –0.3 VRHE and then decreasing. The 50-nm pore can reach a higher peak current density than the 10-nm one, and the difference is greater with a higher r0,1,NRR. Reprinted from Tao et al.115 with permission. Copyright 2021, American Institute of Chemical Engineers. 49 distribution density) and reaction kinetics (e.g., exchange current density and potential), which serves as good guidance in the future design of electrode morphology. 1.4.3.2 Substrate for the Catalyst Layer Deposition. The substrate, planar or porous, can largely affect the macroscopic geometry of the electrode to make it suitable for the transport of reactants in gas or liquid phases. Specifically, the interconnected channels in porous substrates make them the common choice in constructing gas-diffusion electrodes in gas-phase cells so that the triple-phase interface can be properly established. A rotating disk electrode (RDE) made of gold40 or glassy carbon75,114 has been used as a planar platform to uniformly load ENRR catalysts. An RDE normally consists of a non-conductive Teflon casing that encapsulates a mechanically-polished cylindrical conductive disc, exposing a rather small surface area (0.1 – 0.2 cm2) for electrocatalyst deposition.139 Under rotation, the relatively smooth surface area of an RDE helps control the electrode’s surface hydrodynamics to yield insightful information on reaction kinetic parameters (i.e., using the Koutecký-Levich equation) with a lower degree of mass transfer limitation.140 Particularly, rotating an RDE at a high speed can greatly reduce the diffusion layer thickness and quickly detach gas bubbles evolved on the electrode surface under centrifugal force, which collectively minimizes the mass transport overpotential for higher limiting current densities.49 However, one should be aware that the same enhancement of mass transfer kinetics also applies to the N2 and proton fluxes, which does not correspond to an increase in the selectivity toward the ENRR. So far, no comprehensive hydrodynamic study has been conducted concentrating on the competition between ENRR and HER, resembling what has been done for the CO2 reduction as an example.119 Online monitoring of products (NH3/NH4+ and H2) using in-situ surface spectroscopic and/or chromatographic techniques are encouraged to improve the experimental design using RDEs as the platform. 50 On the other hand, metal foam and carbon paper/cloth are porous substrates frequently used to prepare three-dimensional ENRR cathodes. Nickel is a cost-efficient base material to be coated, alloyed, or decorated with precious metal sites to assist N2 adsorption and electron transfer. For example, noble metals (e.g., Pd,141–143 Au,144 and Ru142) can be galvanically deposited on nickel foam which acts both as the substrate and the reductant. This direct deposition of the catalyst layer on a conductive surface removes the influence of binder that needs to be used otherwise in catalyst-coating processes, contributing to a more definitive understanding of the metallic catalyst performance in NH3 electrosynthesis.142 Besides, the direct growth of electrocatalysts on Ni substrates can alter the electric property of the active sites, thus providing a strong catalyst-substrate interaction to improve the ENRR performance. As Li et al.145 proposed, by growing Bi on a Ni foam substrate, not only the poor electroconductivity of Bi can be improved, the low electronegativity of Ni can also contribute to the accumulation of electrons at Bi sites. The facile water splitting on Ni was also considered to facilitate N2 hydrogenation based on the H spill-over from Ni to Bi sites. Similar to Ni, copper foam is another porous metal substrate with excellent electronic conductivity found in self-standing ENRR cathodes.146,147 Carbon-based substrates, such as carbon paper (CP) and carbon cloth (CC), are frequently used in electrocatalysis due to their chemical stability, high electrical conductivity, and expansive surface area to uniformly integrate with metal or non-metal active sites with rich densities. Carbonaceous materials are often considered as a low-cost template capable of generating active sites toward ENRR by introducing dopants, structural defects, and vacancies on a lattice level; therefore, the substrate itself can be chemically modified to become ENRR-active. For example, Li et al.148 observed ENRR activities (rNH₃,geo = 2.59 × 10−10 mol s–1 cmgeo–2 with 6.9% FE) from a heat-treated commercial CC, which were absent from the pristine CC (Fig. 1.16A – B). Cheng et al.149 heated 51 a piece of CC to 800 °C with ammonium persulfate as the source of S and N dopants to create a structurally disordered carbon network with electron-rich doped sites, which improves N2 chemisorption and activation for a good rNH₃,geo of 9.9 x 10–10 mol s–1 cmgeo–2 (Fig. 1.16C). However, the dopants and defective sites of the heat-treated CC can also enhance the HER activity, leading to a low FE of 8.1% that needs to be improved. As mostly found in G-L and G-G cells, porous substrates can act as the gas-diffusion layer (GDL) to improve the mass transfer of gaseous reactants and form a gas-diffusion electrode (GDE) with the deposition of catalyst layer.39,150,151 As illustrated by Fig. 1.16D, the voids in the carbon fibre support make CP/CC substrates gas permeable which means gaseous N2 can diffuse or Fig. 1.16 Effects of catalyst layer substrate to the ENRR in aqueous electrolyte. A – B: comparison between pristine CC and defect CC from heat treatment. Reproduced from Li et al.148 with permission. Copyright 2018, Royal Society of Chemistry. C: ENRR performances from CC after heat-treatment in ammonia persulfate at different temperatures (200, 400, 600, and 800 °C). Reproduced from Cheng et al.149 with permission. Copyright 2020, Royal Society of Chemistry. D: schematic of the cathode gas diffusion electrode (GDE) with porous carbon as the gas diffusion layer (GDL). The insert shows a region where the triple-phase interface is realized within the catalyst layer. E: supplying N2 gas with a tubular GDE shows improved ENRR performance over submerging the cathode in a N2-saturated liquid phase. Reproduced from Wei et al.152 with permission. Copyright 2020, John Wiley and Sons. 52 convectively flow toward the catalyst layer for activation and reduction. As mentioned, the direct contact with N2 gas near the catalyst layer can reduce the N2 mass transfer distance thus its transport resistance, which has shown some promising results in aqueous and non-aqueous ENRR applications. For example, Wei et al.152 fabricated a tubular-shaped GDE using ruthenium-loaded carbon nanotubes and sparged N2 from the inside out (similar to a gas sparger) to obtain an excellent rNH₃,geo of 21 x 10–10 mol s–1 cmgeo–2 with a fair FE of 13.5% at 0 VRHE, which showed a three-fold improvement over the case of bubbling N2 in the electrolyte solution (Fig. 1.16E). Moreover, a microporous layer made of highly conductive and densely packed carbon particles is often added to the GDL, which has been found beneficial in CEM fuel cells to reduce the cell ohmic resistance and improve the reactant/product mass transfer.153,154 However, improving GDEs in NH3 electrosynthesis by adjusting the microporous layer material, pore size, and hydrophobicity has been largely absent. Recently, a three-compartment cell (Fig. 1.4F) that often appears in CO2 reduction applications is also used in NH3 electrosynthesis, where N2 gas and liquid solutions are segregated on the opposite side of the GDE.38,53 With a minimal pressure gradient, N2 crosses the porous substrate to expose more active surface area toward the ENRR with a convectively enhanced triple-phase interface. Wei et al.53 used a ruthenium-based GDE in a three-compartment cell and optimized the electrolyte composition, electrode hydrophobicity, and catalyst mass loading to achieve an excellent FE of 64.8% with an rNH₃,geo of 9.9 x 10–10 mol s–1 cmgeo–2 at –0.1 VRHE. Similarly, using copper-electrodeposited CC as the GDE in a three-compartment cell, Kani et al.38 observed a good rNH₃,geo of 8.6 x 10–10 mol s–1 cmgeo–2 at –0.5 VRHE with an FE of 18% at –0.3 VRHE. 53 1.4.3.3 Binder or Coating Materials Binders are commonly involved in depositing the catalyst layer onto suitable substrates and the type and/or amount of binder can profoundly affect the electrocatalysis performance of the coated electrode.155 Most ENRR studies use the cation conductive ionomer Nafion solution as the binder, whereas in alkaline conditions, an anion conductive ionomer such as the Fumion solution can be applied.50,156 Once present in the catalyst layer, these ionomer solutions can act as an extension of the electrolyte solution or the SPE separator to guide the movement of ions toward or away from the active sites. However, ionomers are generally hydrophilic and tend to lead to H2O flooding and loss of N2 access in GDEs. Hence, PTFE as a hydrophobic binder is sometimes added to the catalyst layer to improve the transportation of gas molecules through the electrode in gas-phase cells. For example, Wei et al.53 deposited a ruthenium-based catalyst layer using PTFE as a binder and after sintering, the hydrophobicity of the GDE increases with the weight percentage of PTFE (Fig. 1.17A). The most hydrophobic GDE showed almost three orders of magnitude higher FE (64.8%) and rNH₃,geo Fig. 1.17 Hydrophobicity engineering on the electrode surface improves the ENRR performance. A: higher levels of hydrophobicity from adding the PTFE binder can improve the rNH₃,geo and FE for a Ru-based GDE. Reproduced from Wei et al.53 with permission. Copyright 2021, American Chemical Society. B: hydrophobic coating can avoid the competing HER for improved N2 mass transfer C: the ENRR performance for ReSe2 coated with hydrophobic carbonized bacterial cellulose. Reproduced from Lai et al.157 with permission. Copyright 2020, John Wiley and Sons. 54 (9.9 x 10–10 mol s–1 cmgeo–2) than the least hydrophobic one. Besides, the maximal rNH₃,geo (10.4 x 10–10 mol s–1 cmgeo–2) was achieved from the electrode with the second-highest PTFE loading, which was attributed to insufficient proton supply blocked by gas bubble coverage. Another option to modify the catalyst layer surface property is to add a hydrophobic coating agent to restrict the proton donor concentration in aqueous conditions so that the HER can be suppressed (Fig. 1.17B). For example, a hydrophobic layer of carbonized bacterial cellulose was laminated on both sides of a catalyst layer made of vacancy-rich ReSe2 and effectively reduced the proton availability from the aqueous electrolyte solution, resulting in a very good FE of 42.5% and fair rNH₃,geo of 4.6 x 10–10 mol s–1 cmgeo–2 in an L-L cell (Fig. 1.17C).157 Likewise, fluorosilane (C11H9F17O3Si) was used to increase the hydrophobicity of a nanoporous Mo4P3-based electrode, which enhanced the FE by almost four times to 10.1% with a fair rNH₃,geo of 2.8 x 10–10 mol s–1 cmgeo–2.158 Also, metal-organic-frameworks (MOFs) and covalent-organic-frameworks (COFs) have been used as a coating material to enhance the electrode’s gas affinity and create an N2-rich microphase. They normally consist of a highly ordered organic molecule network interconnected by nodal metal ions or clusters via coordination or covalent bonding and possess high surface areas from a very porous structure.159 Coating these hydrophobic/aerophilic materials onto a catalyst layer is expected to confine higher amounts of dissolved N2 in numerous pores while restricting the proton supply from electrolyte solutions. Lee et al.40 encapsulated an Ag-deposited gold surface with zeolitic imidazolate framework-71 (ZIF-71) and observed significant suppression on the HER and a fair FE of 18% toward the ENRR. Similarly, Sim et al.159 encapsulated a Pt-deposited Au electrode with ZIF-71 to obtain a good FE of 44% with a yield rate of 161 μg s–1 mgcat−1 (normalized by the catalyst mass loading), and the electronic interaction between ZIF-71 and the 55 Pt nanospheres was found partially responsible for the weakened metal-H bond and hence lowered HER activities. Additionally, from molecular dynamic simulations and diffusion experiments, Liu et al.160 verified the proton-filtering role of a COF as the coating layer, which helped a carbon-based electrode reach an excellent FE of 54.5% with a yield rate of 287 μg s–1 mgcat−1. Lastly, other polymeric organic coatings are also worth studying (e.g., polyaniline161–163 and polyethylene164) given their gas-adsorbing capability similar to the MOF/COF materials. 1.5 Cell Operating Conditions The importance of cell operation conditions should not be overlooked in NH3 electrosynthesis for several reasons. Firstly, tuning operational parameters can optimize the electrolytic cell performance in activity, selectivity, and durability. Secondly, varying operating conditions in ENRR studies helps one to understand the role of individual components and interfacial interactions. Lastly, robust experimental protocols are necessary to minimize false positives. In this section, the effects of temperature, pressure, and gas/liquid flow conditions will be discussed as examples of adjusting the cell operation in NH3 electrosynthesis. Additionally, using operando techniques to validate the ENRR performance and characterize the stability of cell components should be encouraged, preferably by employing multiple in-situ characterization tools. In-situ Fourier-transform infrared spectroscopy,117,132,149,165,166 in-situ X-ray absorption spectroscopy,51 in-situ XRD,103 and differential electrochemical mass spectrometry167,168 are examples that have been applied to elucidate reaction mechanisms in the ENRR and to probe performance losses during operation. 1.5.1 Temperature Controlling the operating temperature in NH3 electrosynthesis needs to balance the thermodynamic and kinetic aspects of the ENRR and the HER. Low temperatures shift the equilibrium toward NH3 56 formation due to the exothermic nature of the ENRR, which also increases the N2 solubility in aqueous or non-aqueous electrolyte solutions. The endothermic H2 formation is inhibited at low temperatures simultaneously. As demonstrated by Guo et al.134, conducting NH3 electrosynthesis with a CoP hollow nanocage catalyst at 25 °C generated nearly three times higher rNH₃,geo and FE than running the experiment at elevated temperatures (Fig. 1.18A). From a kinetic perspective, elevated temperatures can increase reaction rate constants according to the Arrhenius equation, which in theory improves the reaction rate for both the ENRR and the HER. Additionally, as the molecular mobility increases with temperature, the ion conductivity and N2 diffusivity in electrolyte solutions can be enhanced, which potentially contributes to higher fluxes of reactants toward active sites.169–171 The above kinetic benefits have been observed in ENRR studies that raised the cell temperature above room temperature.58,73,75,133,172–175 However, the reaction rate for the HER is simultaneously improved in heated conditions, which can compete with the N2 chemisorption and reduction and diminish the electrocatalytic surface with excessive bubble formation, resulting in compromised reaction selectivity. For instance, using a LaFeO3-based perovskite catalyst, Zhang et al.176 observed a monotonic increase in rNH₃,geo from 40 to 80 Fig. 1.18 The effects of operating temperature on the NH3 electrosynthesis. A: hollow CoP nanocages show the highest results at room temperature rather than elevated temperatures in a L-L cell. Reproduced from Guo et al.134 with permission. Copyright 2021, American Chemical Society. B: peak performances were obtained at 80 °C before decreasing at 100 °C for a LaFeO3-based perovskite catalyst. Reproduced from Zhang et al.176 with permission. Copyright 2019, American Chemical Society. C: N-doped carbon shows opposite temperature dependence for the NH3 yield in alkaline and in acid electrolyte solutions. Reproduced from Mukherjee et al.75 with permission. Copyright 2018, Elsevier B.V. 57 °C (0.9 and 3.3 x 10–10 mol s–1 cmgeo–2, respectively) which decreased to 2.3 x 10–10 mol s–1 cmgeo–2 at 100 °C along with a four-fold drop in FE (Fig. 1.18B). The balance between the ENRR and the HER in response to the cell operating temperature can be affected by the cell component design such as the pH of electrolyte solutions. For example, using nitrogen-doped carbon as the catalyst, Mukherjee et al.75 observed an increase in rNH₃,geo with temperature in 0.1 M KOH solution but the reverse in 0.1 M HCl solution (Fig. 1.18C). A lower kinetic barrier for the HER to proceed in acidic than in alkaline conditions was considered as the reason for the depletion of catalytic sites toward N2 adsorption and activation. 1.5.2 Pressure For liquid phase cells, the N2 partial pressure in the overhead space linearly correlates with the N2 solubility in liquid electrolyte solutions as stated by Henry’s law. According to Chatelier’s principle, a higher pressure also shifts the ENRR equilibrium in favour of NH3 formation considering its volume-decreasing property but has an opposite effect on the H2 formation.26 For example, Cheng et al.177 pressurized a liquid phase cell from 0.1 MPa up to 0.7 MPa and observed an increase in rNH₃,geo up to 2.5 x 10 –10 mol s–1 cmgeo–2 at –0.025 VRHE from a Fe3Mo3C/C catalyst (Fig. 1.19A – B). Similarly, using a rhodium-based single-atom catalyst, Zou et al.178 increased the cell pressure to 5.5 MPa for an excellent rNH₃,geo of 12.1 x 10–10 mol s–1 cmgeo–2 and good FE of 20.4% at –0.2 VRHE, which was 5 – 7 times higher than the results at ambient pressure at the same potential. However, how to supply and store N2 to a pressurized system introduces new engineering challenges such as the cost and physical integrity of the cell components. Therefore, the ideal case for the deployment of aqueous ENRR for decentralized NH3 production would require a system that performs mainly at the ambient pressure. 58 In gas-phase electrolytic cells, the pressure distribution across the GDE is influenced by multiple factors such as the geometric design of the flow channel geometry (e.g., interdigitated or serpentine), liquid presence, the architectures of the electrode, etc. Rigorously controlling these parameters to achieve a uniform reactant distribution requires fluid dynamic simulations and pressure monitoring which have not been thoroughly investigated in NH3 electrosynthesis. The N2 partial pressure in the gas supply and pressure drop across the cell are only two parameters that have been preliminarily investigated. For example, Kani et al.38 observed a monotonic decrease in the FE and rNH₃,geo on a copper-based GDE with increasing percentages of argon that was blended Fig. 1.19 The effects of operating pressure on the NH3 electrosynthesis. A – B: performance optimization with applied potentials and the gauge pressure in an air-tight NS cell. Reprinted from Cheng et al.177 with permission. Copyright 2019, John Wiley and Sons. C: the FE and rNH₃,geo both increase monotonically with a higher mole fraction of N2 in the gas supply blended with Ar. Reprinted from Kani et al.38 with permission. Copyright 2020, American Chemical Society. D: illustration of a four-compartment cell with a gas-liquid-liquid-gas (G-L-L-G) symmetric configuration. E: the jNH₃ and FE decreases when the N2 supply was diluted with Ar in the Li-mediated ENRR with a total pressure of 1 bar. Reproduced from Lazouski et al.66 with permission. Copyright 2019, Elsevier Inc. F: the measured (symbols) and predicted (lines) FE as a function of nitrogen pressure at 1 vol% fixed EtOH concentration. The red and blue colours represent the constant and cyclic Li deposition strategies, respectively. The green vertical line divides the N2 and proton mass transfer limited regimes. The shaded area covers ±50% uncertainties for each line. Reprinted from Andersen et al.105 with permission. Copyright 2020, Royal Society of Chemistry. 59 into the N2 gas supply (Fig. 1.19C). The jNH₃ was fitted to a linear relationship with the N2 mole fraction, which suggests the N2 hydronation being the rate-limiting step. In Li-mediated non-aqueous NH3 electrosynthesis, the gas pressure was also found to be relevant in achieving stable operation with high NH3 yields. For example, Lazouski et al.39 applied a four-compartment cell by incorporating a gas- and a liquid-phase compartment symmetrically on both the cathode and anode side (Fig. 1.19D). The N2 and H2 gas flow needed to be supplied with a minimum of 0.5 kPa pressure gradient across the stainless-steel cloth electrodes to prevent the complete electrolyte flooding and therefore the loss of ENRR and HOR activities. In a non-aqueous L-L cell, Lazouski et al.66 studied the effect of N2 partial pressure by diluting the gas supply with argon, which negatively affects the FE and rNH₃,geo toward the NH3 formation (Fig. 1.19E) under 1 bar. Moreover, Andersen et al.105 evaluated the FE obtained from the Li-mediated ENRR with a varied N2 total pressure in an NS cell (Fig. 1.19F). An increased cell pressure from 1 bar to 10 bar improves the FE significantly but such enhancement diminishes when further pressurizing the cell from 10 bar to 20 bar, which was ascribed to sufficient N2 mass transfer at 10 bar and above not being the rate-limiting factor. On the other hand, the proton donor concentration (1 vol% EtOH) was assumed to become mass-transfer limited at elevated N2 pressures as the Li3N produced from the Li nitridation cannot be rapidly hydrolysed to form NH3. Note that the mass flow rate or the total mass of N2 gas is increased with higher N2 pressures if the volumetric flow rates or total volume are held constant. Proportionally, impurities from the gas supply can infiltrate the system with higher quantities, resulting in higher concentrations of apparent NH3 obtained from the electrolysis and falsely attributed performance improvement from the pressure change. Therefore, assessing the overall contamination content from the gas supply is necessary for studies that adjust the gas pressure as a strategy to boost the ENRR results. 60 1.5.3 Gas and Liquid Flow Conditions The volumetric flow rate is commonly adjusted to control the gas flow condition in an electrolytic cell to possibly improve the ENRR performance. For a flooded electrode surface, increasing N2 flow rates has no influence on N2 solubility in the electrolyte solution, as evidenced by studies that reported no apparent change in rNH₃,geo or FE with a higher N2 dispensing rate.179–183 However, the agitation from a more vigorous gas bubbling may enhance the convection at the electrode surface, thus likely to decrease the diffusion layer thickness to improve the N2 mass transfer. For example, Wang et al.184 observed higher FE and rNH₃,geo results from a palladium-loaded CC by increasing the gas flow rate (50, 80, 100, and 130 standard cubic centimetre per minute [sccm]), which is more pronounced when the gas bubbler is positioned closer to the electrode (Fig. 1.20A – B). Such improvement was attributed to the more frequent contact between gas bubbles and the electrode, which partially encapsulates the electrode with N2 gas and essentially realizes the triple-phase interface locally. For electrodes that are exposed directly to the gas phase, studies that investigate the effects of gas flow rate have been largely absent. From a three-compartment cell (Fig. 1.4F), Kani et al.38 showed that increasing the N2 gas flow rate (from 40 to 150 sccm) can reduce the boundary layer thickness of the electrolyte at the GDE surface and increase the dissolved N2 concentration locally, which improves the rNH₃,geo and FE toward the ENRR (Fig. 1.20C – D). In cells that contain a liquid compartment, the liquid electrolyte solution can remain stagnant throughout an electrolysis test without refreshing, or be recirculated in a continuous-flow mode under forced convection (e.g., using a peristaltic pump).185–188 The liquid electrolyte is often recirculated back to the same chamber instead of being single-passed to allow for the accumulation of NH4+ product in the solution. Similar to the agitating role of the gaseous component, the convective force from liquid recirculation can enhance the mass transport of reactant species and 61 prevent local effects from reactant consumption or product accumulation, which has been considered vital for cells in a continuous-flow configuration.53,189 Furthermore, the advantage of forced convection can be fully realized by dispersing solid catalyst particles in the recirculating liquid stream to improve the N2 mass transfer and utilization of electrocatalyst. As illustrated in Fig. 1.21A, the dispersed particles are normally designed to be able to facilely chemisorb dissolved N2 or have a hydrophobic surface to coalesce N2 gas bubbles, and when colliding with a static current collector or a catalyst-coated electrode, electrons can be Fig. 1.20 Effects of gas flow rate to the ENRR performance. A – B: increasing the gas bubbling rate in L-L cells improves the ENRR performance particularly the FE, more so if the bubbler was positioned closer to the electrode catalyst layer. Reproduced under a Creative Commons Attribution License (CC BY 4.0). Copyright 2019, Wang et al.196 C – D: higher N2 gas flow rates can improve the FE and rNH₃,geo, due to the reduced boundary layer thickness and higher dissolved N2 concentration within the GDE. Reproduced from Kani et al.38 with permission. Copyright 2020, American Chemical Society. 62 delivered to the surface-bound N2 for the subsequent activation and reduction. For example, with a titanium mesh as the current collector, Li et al.190 dispersed silver nanodots in a 0.1 M Na2SO4 solution that was under recirculation at 10 mL min–1. The small and non-aggregating silver nanodots dispersed in the electrolyte can provide an abundant surface area to adsorb and activate N2, resulting in higher ENRR performances than the conventional electrode-bound catalyst Fig. 1.21 Recirculated electrolyte containing electrocatalysts as a design strategy to improve the ENRR performance. A: schematic of a non-loading electrocatalytic system that can adsorb N2 efficiently and transport to the current collector under convective flow. B: dispersing Ag nanodots in N2-saturated 0.1 M Na2SO4 solution can obtain higher NH3 yield (normalized by the catalyst mass loading) and FE, in comparison with electrodes with immobilized Ag nanodots, Ag2O nanoparticles, and Ag nanoparticles. Reproduced under a Creative Commons Attribution License (CC BY 4.0). Copyright 2021, Li et al.190 C: the suspended aerophilic Bi5O7I nanotubes can improve the NH3 electrosynthesis with room for optimization. Reprinted from Liu et al.191 with permission. Copyright 2020, Elsevier B.V. 63 utilization. At –0.25 VRHE, the NH3 production rate obtained from the catalyst-in-solution system was much higher than the catalyst-coated electrode along with a better FE (Fig. 1.21B). Similarly, Liu et al.191 dispersed hydrophobic Bi5O7I nanotubes in a recirculated electrolyte solution to actively transport N2 gas toward a catalyst-coated electrode that contains both hydrophilic carbon spheres and Bi5O7I. An aerophilic-hydrophilic heterostructure is then formed at the electrode-electrolyte interface to efficiently enrich the local N2 availability. Consequently, both the rNH₃,geo and the FE increase as more Bi5O7I nanotubes are added to the electrolyte (Fig. 1.21C). Interestingly, when electrolyte recirculation (188.4 mL min–1) is provided to the system, the NH2 yield can be further enhanced despite a lower FE toward the ENRR. 1.6 Introduction Summary and Outlook Existing studies that emphasize on the electrocatalyst development have provided insights on how to reduce the kinetic barrier of activating and reducing N2 while suppressing the formation of H2. However, the electrocatalyst design as a singular research objective may have resulted in overlooking the importance of cell design and operational conditions. As summarized in this review chapter, innovative designs of the cell layout, separator material, electrolyte composition, and electrode architecture are increasingly proposed to fulfil the potential of the electrocatalyst. Enhancing the performance in aqueous or non-aqueous NH3 electrosynthesis by adjusting the operating temperature, pressure, and reactant flow configuration are also attempted. To advance, the following knowledge gaps/ issues remain critical and require additional attention: 1. The cathode architecture should be optimized with emphasis on the interfacial design, such that advanced ENRR electrocatalysts can be properly developed and integrated. In this regard, expanding the triple-phase interface can be beneficial to expose more active sites for the N2 chemisorption and activation. Currently, adjusting the wettability of the catalyst layer by 64 incorporating binders or coating hydrophobic materials with optimized ratios has shown promise in liquid phase cells in suppressing the HER, whereas loading the catalyst layer on porous substrates as GDEs is viewed as an advantage for gas-phase cells. Moving forward, both control strategies for the gas and liquid phases should be considered. In practice, cells with multiple compartments to host reactants in various phases (e.g., gas-liquid-liquid three-compartment cell shown in Fig. 1.4F and Fig. 1.20D) can be explored, so that the relative availability of N2 and proton can be regulated from the gas and liquid side of an electrode independently. 2. Improved kinetic and thermodynamic understanding of Li-mediated non-aqueous cells that are operated continuously is needed to exploit the potential of this viable and possibly scalable route for NH3 electrosynthesis. Notably, the cell design and operational optimization in Li-mediated systems have shown a significant if not essential role, which differs from most aqueous systems where the electrocatalyst development is considered as the focus. Adopting in-situ characterization techniques and insights from aqueous or non-aqueous Li-ion batteries (e.g., immiscible liquids, artificial SEI layers, microporous or Li+-conducting separators, etc.) can be useful in this regard. 3. Last but not the least, rigorous testing protocols especially measuring the background N before and during the electrolysis remain important in future ENRR studies. Thorough cleaning of the electrodes, electrolyte, and gas supply should be conducted prior to benchmarking ENRR performances of newly-designed electrocatalysts or cell designs. Appropriate control experiments, repeatable qualitative/quantitative NMR tests with purified 15N2 gas supply, and in-situ surface characterization techniques are recommended to identify and eliminate false positive results in this field. 65 Chapter 2 Experimental 2.1 Flow Cell Electrolyzer for the ENRR in Aqueous Media The same electrolyzer setup was used for Chapter 3 and Chapter 4 toward the aqueous ENRR, despite differences in the flow configuration and electrochemical characterization techniques. Those distinctions are highlighted in Section 2.1.3 and Section 2.1.4. 2.1.1 Electrode preparation To prepare the membrane separator for catalyst spraying, Nafion N112 and N350 membranes (DuPont Inc.) were boiled at ca. 80 °C for 1 h each in 3% H2O2, deionized (DI) H2O, and 0.5 M H2SO4 to remove impurities. The membrane was rinsed with DI H2O between boiling steps and after the final boiling step. The membrane was then dried on a flat surface in an oven at ca. 40 °C and ambient pressure overnight. The anion exchange membrane (AEM, X37-50 Grade RT) from Sustainion and microporous diaphragms from Celgard (C3400, C3500, and C5550) are used directly for the catalyst casting without pre-treatment. A programmable spray coater (FlexiCoat OP3, Sono-tek) equipped with a sonicating syringe pump and an ultrasonic nozzle was used to prepare the electrodes (anode and cathode) with a uniform catalyst mass loading of 1.0 mg cm-2. For the cathode gas diffusion electrode (GDE), 40 wt% Au on Ketjenblack 300 (Au/KB) catalyst ink from Fuel Cell Store was coated on a 5-cm2 carbon paper (AvCarb GDS3215, wet-proofed, Fuel Cell Store) at 60 °C. 40 wt% Pt on Vulcan XC72R (Pt/XC) from Fuel Cell Earth was sprayed onto the cleaned Nafion membrane or alternative separator materials at 60 °C as the anode catalyst-coated membrane (CCM). The electrocatalyst ink (Au/KB or Pt/XC) also contains a mixture of isopropanol and DI H2O (v/v = 1:0.7) as the solvent and Nafion ionomer (5 wt% in H2O, DuPont Inc.) as the binder to achieve a final ionomer content of 32 wt% in the catalyst layer. Commercial GDEs were purchased from Fuel Cell Store that consists 66 of 60 wt% Au supported by Vulcan XC72C (Au/XC, loading = 1 mg cm−2) and GDS3215 as the substrate, which serves as the alternative to homemade Au/KB GDEs. Similarly, commercial CCMs (Johnson Matthey) using Pt black (Pt-CCMs, loading: 0.4 mgPt cm−2) were also purchased as the alternative to homemade Pt/XC CCMs. Apart from the Au-based cathode GDE, a GDE deposited with MoS2 nanosheet clusters was also prepared by the facile hydrothermal synthesis method modified from the literature.192 100 mg of thiourea (Sigma Aldrich) and 100 mg sodium molybdate (Sigma Aldrich) were dissolved in 15 mL DI H2O and stirred for 30 min. The resulting solution was transferred to a 20-mL Teflon-lined autoclave where a piece of rectangular carbon paper substrate (AvCarb GDS3215, 2.5 cm x 4 cm) was also inserted. The autoclave was then sealed and positioned in the center of a temperature-programmable muffle furnace. The furnace was heated from room temperature to 200 °C at a ramping rate of 5 °C min−1 before holding at 200 °C for 24 h. The autoclave was then cooled down in the furnace naturally before opening in the fume hood. The resulting MoS2-coated GDE was rinsed thoroughly with copious IPA and DI H2O and stored in 0.5 M H2SO4 to completely remove any residual thiourea as a possible nitrogenous contamination source. The mass loading of MoS2 was calculated as 0.5 mg cm−2 based on the weight difference before and after the hydrothermal synthesis. 67 2.1.2 Physical Characterization The polycrystalline Au/KB catalyst was characterized by powder X-ray diffraction (PXRD, D2 Phaser Diffractometer, Bruker) using a copper Kα radiation source and a LynxeyeTM detector (Fig. 2.1A). The distribution of Au nanoparticles on the Ketjenblack carbon was observed by transmission electron microscopy (TEM, FEI Tecnai G2 200 kV) in Fig. 2.1B. Selected area electron diffraction (SAED) was performed on the TEM to confirm the polycrystallinity of Au (Fig. 2.1C). As shown in Fig. 2.2, the sprayed Au GDE was also characterized before and after crossover experiments with a scanning electron microscope (SEM, HITACHI S2600 Variable Fig. 2.1 Physical characterization of the Au/KB catalyst ink used for the cathode catalyst layer. A: X-ray diffraction (XRD) pattern of the polycrystalline Au and comparison with crystallography open database (COD). B: selected area electron diffraction (SAED) on the polycrystalline Au. C: transmission electron microscopy (TEM) images of the Au/KB catalyst. D: particle size distribution of the Au nanoparticles in the Au/KB catalyst. 68 Pressure) and elemental mapping was conducted using the energy dispersive X-ray (EDX) detector. The Pt loading on the CCM anode was quantified by X-ray fluorescence spectroscopy (XRF, Fischerscope X-Ray XDV-SDD) in Fig. 2.3. Fig. 2.2 Physical characterization of the Au/KB cathode GDE. A – B: EDX spectroscopy analysis of the elemental composition of the GDE before and after the NH4+ crossover experiment, respectively. C: EDX mapping of different elements (C, F, and Au) as well as the overlapped image. 69 The MoS cathode GDE prepared from hydrothermal synthesis was also characterized with the SEM and TEM. As shown in Fig. 2.4A – C, clusters of nanosheets in the spherical shape were observed, which are stably attached to the carbon fibre support or the microporous carbon layer of the GDS3215 carbon paper substrate. TEM images (Fig. 2.4D – E) revealed transparent nanosheets at the edge of spherical clusters and the wrinkled morphology indicates a thin layered structure. To confirm the elemental distribution of MoS2 on the GDE, XRD and EDX were performance with the prepared MoS2 cathode GDE. As shown in Fig. 2.5A, the diffraction pattern of the MoS2-loaded GDE demonstrates peaks for both the graphite substrate at ca. 26.5° and 57.5°, and MoS2 (2H phase) at ca. 33°, 34°, 40° and 59°. Additionally, bulk MoS2 powder (American Element) was also examined for the XRD as a reference material which validated the presence of MoS2 on the prepared cathode GDE. In addition, Fig. 2.5B shows the EDX elemental mapping that confirms the surface deposition of Mo (30 wt%) and S (5.6 wt%) on the carbon substrate (63.7 wt%). A B Fig. 2.3 A: Pt loading on the Nafion membrane measured by XRF. B: The frame used to cover the Nafion substrate for the spray coating process. Dimension unit: mm. 70 Lastly, the wettability of the Celgard diaphragms was evaluated based on the static contact angle. The contact angle measurement was taken by dropping 10 µL DI H2O to the separator that is taped onto a flat surface. Images were taken using FTA121110-Falcon setup equipped with an LED backlight. FTA32 software was used to fit the droplet curvature and calculate the contact angle. Three contact angle measurements were taken for each Celgard separator. Fig. 2.4 Characterization of MoS2 on the cathode GDE prepared from the hydrothermal synthesis process. A – C: SEM images of MoS2 spherical clusters at different scales. D – E: TEM images of MoS2 nanosheets at different scales. 71 2.1.3 Electrolyzer Cell and Flow Configurations The prepared 5-cm2 cathode GDE and anode CCM were pneumatically compressed in a customized electrolyzer cell (TP5E, Tandem Technologies) at 85 psig (5.86 barg) to form a membrane electrode assembly (MEA), where the anode was positioned above the cathode ( 𝐻2(𝑔) → 2𝐻+(𝑎𝑞) + 2 𝑒﷩− 𝐸𝑒𝑞° = 0 𝑉﷩𝑆𝐻𝐸﷩ (2.1) 𝑂2(𝑔) + 4𝐻+(𝑎𝑞) + 4𝑒− →2𝐻2𝑂(𝑙) 𝐸𝑒𝑞° = 1.23 𝑉𝑆𝐻𝐸 (2.2) and Fig. B.1). To evaluate the ENRR performance of Au-based electrocatalysts on the cathode GDE (Chapter 3), Ar or N2 gas was supplied to the cathode side at 50 sccm as shown in Fig. 2.7A. Despite a high purity (99.999%), the inlet gas was still purified by passing through a packed-bed NOx scrubber (Bacharach Inc.) followed by an acid scrubber (0.05 M H2SO4). For the anode side, H2 gas (purity = 99.999%) flowing at 50 sccm passes through an acid scrubber which also humidifies the gas at room temperature (20 ± 1 °C). The hydrogen oxidation reaction (HOR, equation 2.1) was used as the anodic reaction to prevent carbon corrosion and lower the energy Fig. 2.5 Elemental analysis for the MoS2 deposited onto carbon paper substrate as the cathode GDE. A: XRD for MoS2-GDE and bulk MoS2 powder as the reference. B: EDX analysis of Mo, S, and C. 72 consumption as opposed to the kinetically sluggish oxygen evolution reaction (OER, equation 2.2) which requires a much higher overpotential.193 𝐻2(𝑔) → 2𝐻+(𝑎𝑞) + 2𝑒− 𝐸𝑒𝑞° = 0 𝑉𝑆𝐻𝐸 (2.1) 𝑂2(𝑔) + 4𝐻+(𝑎𝑞) + 4𝑒− → 2𝐻2𝑂(𝑙) 𝐸𝑒𝑞° = 1.23 𝑉𝑆𝐻𝐸 (2.2) Fig. 2.6 Exploded view of the prismatic electrolyzer cell containing a catalyst-coated membrane (CCM) for the anode and a gas diffusion electrode (GDE) for the cathode. 73 Fig. 2.7 Illustrations of the liquid and gas flow configurations in the electrolytic system for A: ENRR performance evaluation and B: NH4+ crossover assessment. Samples from the gas-liquid separators and the acid absorbers were collected for NH4+ quantification. 74 Approximately 15 mL of liquid electrolyte (0.1 M Na2SO4) was recirculated on the cathode side via a peristaltic pump at 100 rpm (equiv. 4 mL min−1) to collect NH4+ from the cathode GDE so that the 𝑐𝑁𝐻4+ can be concentrated to detectable levels (> 0.05 ppm). Otherwise, the concentration would be too low with single-pass electrolyte flow. For the anode side, no aqueous electrolyte was recirculated (H2 gas only) since no NH4+ was expected to enter the anode compartment based on the assumption of the separator (Nafion 112) being able to prevent any NH4+ crossover. However, as discussed later in Chapter 4, NH4+ crossover does occur for common separators at 𝑐𝑁𝐻4+ that are relevant to the ENRR results, contributing to the false positives in aqueous media. Therefore, to assess the reliability of different separators under various operating conditions (Chapter 4), the gas-liquid co-flow configuration was used for the crossover investigation in both the cathode and anode compartments with the same electrolyte composition and recirculation rate (Fig. 2.7B). In addition, Ar gas was supplied to the cathode instead of N2 to focus on elucidating the NH4+ crossover when only the HER is considered to occur on the cathode. For cases where the gas-liquid co-flow configuration was used, the single-pass gas supply and the recirculated liquid electrolyte merge at a tee connector before entering a serpentine graphite flow field (channel area = 2.3 cm2). The flow field channel is 0.86 mm in width and 1.10 mm in depth, whereas the landing is 0.65 mm in width. At the flow field outlet, the gas-liquid co-flow stream passes through a gas-liquid separator. The gas in the headspace enters two consecutive acid absorbers (0.05 M H2SO4) to remove remnant NH3/NH4+ in the exhaust from both the cathode and anode compartments. The first acid absorber (ca. 6 mL) was sufficient to capture all the NH4+ in the exhaust gas unless high 𝑐𝑁𝐻4+ (≥ 100 ppm) were used for the crossover tests. No noticeable NH4+ could be detected from the second acid absorber (25 mL) for most cases but was still included 75 in the process train. Negligible back pressure (ca. 0.2 psi) was provided at the gas outlet (Fig. B.2). All experiments were performed at room temperature unless specified otherwise. All area normalization references the active geometric area of the MEA (5 cm2). In term of the form of final NH3 products, gaseous NH3 carried out by the exhaust gas is desired as downstream separation could be avoided as compared to the dissolved NH3 or acidified NH4+ in the electrolyte, especially for the non-aqueous environment where the volatile solvent (i.e., THF) would require costly processes for its removal. Therefore, the larger proportion of NH3 is captured by the acid absorbers during the ENRR experiments as opposed to the dissolved NH3 or solvated NH4+ in the catholyte, the more energetically favorable the electrolytic systems become. 2.1.4 Electrochemical measurements A two-electrode configuration was used in the electrolytic systems by connecting with a Biologic potentiostat (VMP-3, Biologic Science Instruments). The Au/KB or MoS2 GDE was used as the cathode and the working electrode (WE), whereas a Pt-based CCM was used as the anode and the counter electrode (CE). Au and MoS2 were selected for the cathode given its frequent appearances in previous ENRR studies.43,133,194–196 H2 gas was supplied to the anode to facilitate the HOR, which has negligible overpotential on the Pt anode at a low current density. Therefore, the Pt anode can be expected to also function as a pseudo-reference electrode (RE). For the ENRR performance evaluation, linear scan voltammetry (LSV) at 5 mV s−1 scan rates was conducted from 0 – −1.0 V 10 times with N2 or Ar supplied to the cathode. 0.1 M Na2SO4 was used as the electrolyte with a co-flow of N2 or Ar to the cathode. Open circuit voltage (OCV) was performed before each LSV scan for stabilization. The 10th cycle from the N2- or Ar-saturated LSV were compared to select the appropriate voltage window – regions where the current density is higher for the N2 case that the Ar counterpart – to perform the subsequent potentiostatic 76 polarization electrolysis. Potentiostatic polarization tests were conducted for 4-h with N2-saturated electrolyte. In addition, the following control experiments were conducted: (1) potentiostatic polarization tests with Ar-saturated electrolyte; (2) OCV tests with N2-saturated electrolyte. Those control experimented are expected to yield negligible NH4+ compared to the PP tests with N2-saturated electrolyte. For the crossover experiment in Chapter 4, OCV or galvanostatic polarization (GP) at 𝑗 = −0.1 – −10 mA cm−2 (equiv. −1 – −100 A m−2) were conducted. The low current density (−0.1 mA cm−2) was of special interests in order to represent operating conditions in the previous ENRR studies that can effectively avoid excessive HER.197 Such a low current density level should be able to establish an electric field without significant side reactions, such as NOx reduction, NH3/NH4+ oxidation, and/or carbon corrosion, that may skew the objective of observing the NH4+ crossover. Cyclic voltammetry (CV) at a scan rate of 100 mV s−1 within 0 – 1.65 V in cell voltage (vs the anode potential) was performed before and after each crossover test. The results and chronological changes in the CV scans serve as a diagnostic tool to track physiochemical changes to the MEA. See Section 2.1.6 and Appendix B for details. Electrochemical impedance spectroscopy (EIS) at OCV from 100 kHz to 100 mHz was conducted with the high frequency intercept at the real axis used as the ohmic resistance for manual ohmic loss (𝑖𝑅) correction. 2.1.5 NH4+ Crossover Tests To investigate the effectiveness of Nafion membrane in preventing NH4+ crossover in a flow electrolysis cell, 1 ppm NH4+ was added either to the catholyte (cathode-fed), or anolyte (anode-fed), or both sides (symmetric fed), which forms an initial concentration gradient (Δ𝑐𝑁𝐻4+ = 𝑐𝐶𝐴 −𝑐𝐴𝑁 ) of 1, −1, and 0 ppm, respectively. Over 7 h, the electrolyte was sampled and refilled periodically to measure the remaining NH4+ concentration in the catholyte (𝑐𝐶𝐴) and anolyte (𝑐𝐴𝑁). 77 After each crossover test, the system was rinsed with 1 L DI H2O in a single pass followed by recirculating 1 L fresh DI H2O overnight. 1 ppm NH4+ has been used in most previous NH4+ crossover studies conducted in H-cells 46,198,199 and using the same net concentration gradients can allow meaningful comparisons with our flow electrolysis cell. In addition, 1 ppm NH4+ has been reported in laboratory-scale ENRR tests using state-of-the-art electrocatalysts 200–204 and is well above the lower detection limits of NH4+ using state of the art analytical quantification methods.29,31 Hence, changes to the NH4+ content at this concentration level is representative of the current research progress towards NH3 electrosynthesis from N2 in aqueous media and can be reliably quantified. Nafion N112 was used for the crossover experiment with varied electrolyte compositions. H2SO4 (0.05 M, pH = 1), NaOH (0.1 M, pH = 13), or Na2SO4 (0.1 M, pH = ca. 6) were used as the acidic, alkaline, and neutral supporting electrolyte, respectively, which all frequently appear in previous ENRR studies. DI H2O with no supporting electrolyte (pH = ca. 7) was also tested. Notably, DI H2O was shown by Lan et al.205 to achieve one of the highest NH4+ production rates (1.14 x 10 −5 mol m−2 s−1) to-date in a flow electrolysis cell. To match the anionic species solutions, (NH4)2SO4 was used as the NH4+ source for the H2SO4 and Na2SO4 electrolyte, and NH4OH for the NaOH and H2O electrolyte. Chemicals used for the electrolyte preparation were all purchased from Sigma-Aldrich Corp. with purities at least of the ACS grade. The NH4+ crossover with Nafion N112 membrane was also tested at higher 𝑐𝑁𝐻4+ (1, 10, 100, and 3400 ppm or equiv. 0.2 M) with 0.1 M Na2SO4 as the background electrolyte, and higher current density (−0.1 – −10 mA cm−2) to investigate the effect of operation conditions. Alternative separator materials were also tested for 1-ppm NH4+ crossover, including AEM (Sustainion, X37-50 Grade RT), microporous diaphragms (Celgard, C3400, C3500, and C5550), and Nafion N350 78 (thickness = 254 µm) as a thicker alternative to N112 (thickness = 50.8 µm). The physical properties of those separator materials are included in Table 2.1. Crossover tests in each type of electrolyte were conducted using two separately prepared MEAs, which yielded comparable results consistently if appropriate conditioning was followed. 79 Table 2.1 Properties of Different Separator Materials. Separator type Cation-exchange membrane Anion-exchange membrane Microporous diaphragm N112 N350 X37-50 Grade RT C3400 C3500 C5550 Material A polymer containing a PTFE backbone and ether ketone side chains with sulfonate end groups54 A imidazolium- or pyridinium-functionalized styrene polymer206 Polypropylene207 Supplier DuPont Sustainion Celgard Thickness, μm 50.8 254 50 25 25 102 (microporous diaphragm thickness = 25 μm) Ion exchange capacity (IEC), meq g−1 0.92 0.90 1.2 N/A N/A N/A Porosity, % N/A N/A N/A 41 55 55 (microporous diaphragm) Surface modification N/A N/A N/A Surfactant-coated Surfactant-coated Surfactant-coated, laminated to a non-woven polypropylene fabric 80 2.1.6 MEA conditioning for NH4+ crossover in Na2SO4 In H2SO4, NaOH, and H2O, similar CV scans were observed from a freshly installed MEA and after NH4+ crossover tests were completed in each type of electrolyte (Fig. B.3). Such similar results suggest a negligible influence from the supporting electrolyte (and NH4+) on the MEA in those electrolyte solutions. However, the peaks for the Au and quinone redox couples in the voltammogram decrease significantly in Na2SO4 as repeated crossover tests were conducted. Nafion is known to be suspectable to cation adsorption including Na+, which leads to reduced ionic conductivity in the membrane and/or depleted H+ availability in the catalyst layer during electrochemical operations,208,209 which may explain in the observed variations in CV scans for the Na2SO4 case. (See Appendix B Diagnostics on the Status of MEA based on the Cyclic Voltammetry.) Therefore, to ensure consistent electrochemical and physiochemical properties of the MEA in Na2SO4, MEA conditioning was conducted by soaking the cathode GDE and anode CCM in 15 mL of 2 M Na2SO4 solution for ≥ 24 h and then transferring it to 15 mL 0.1 M Na2SO4 solution as storage for ≥ 24 h until the MEA was assembled. Such a protocol was described by Evans et al.210 to alter and maintain uniform membrane morphology (i.e., equivalent pore sizes) in given electrolyte salts including Na+. Additionally, the soaking procedure also demonstrates the capability to remove NH4+ and NO3− contaminants from the as-prepared MEA components. For example, a 5-cm2 as-received N112 membrane desorbed 1.42 ± 0.09 µg NH4+ and 1.49 ± 0.09 µg NO3− to 15 mL 2 M Na2SO4 solution and repeating the same soaking protocol results in less than 0.1 µg NH4+ or NO3− which is well below detection limits. Besides soaking the MEA in concentrated Na2SO4 solutions, electrolysis conditioning was also attempted to completely remove contaminants from freshly prepared MEAs (with Nafion or alternative separators) and achieve consistent results. Each MEA was conditioned by electrolysis 81 at −10 A m−2 for 8 h after being installed into the electrolyzer. Blank Na2SO4 was added as the electrolyte and sampled for 𝑐𝑁𝐻4+ every 2 h. Ar and H2 gases were supplied to cathode and anode side, respectively, at the same flow rate as crossover tests. The electrolysis conditioning was repeated at least twice until the detected NH4+ becomes consistently low (below 0.05 ppm) or catholyte pH increases can be consistently observed as an indication of complete H+ consumption in the separator. 2.2 Flow Cell Electrolyzer for the ENRR in Non-aqueous Media 2.2.1 Electrode preparation and characterization 316 stainless steel cloth (SSC) mesh from McMaster-Carr (400 x 400 mesh size) with 25 µm wire diameter and 38% opening was used as the substrate. The SSC was consecutively sonicated in isopropanol (IPA) and deionized (DI) H2O for 30 min each to remove surface impurities before drying at 80 °C for 1 h. A programmable spray coater (FlexiCoat OP3, Sono-tek) equipped with an ultrasonic nozzle was used to brush the catalyst ink to each side of the SSC for a mass loading of 2.0 ± 0.1 mg cm−2. The sprayed electrodes were dried at 120 °C under vacuum overnight and then stored in the Ar-filled glove box to keep the moisture low (< 0.5 ppm). Prior to each test, the sprayed electrode was hot-pressed at 80 °C and 1500 psig (ca. 1 MPa) for 20 min with a hydraulic hot press (Carver Inc.). The void (38 µm in width) of the mesh was filled by the sprayed reaction layer (RL) after hot-pressing, which assists with avoiding possible electrolyte crossover to the gas chambers. Vulcan XC72R-supported Pt (Pt/XC, Fuel Cell Earth), Vulcan XC72R (XC, Cabot), C-NERGY SUPER C45 Conductive Carbon Black (C45, MTI Corp.), and C-NERGY SFG6L Graphite (SFG, MTI Corp.) were used in the sprayed ink. XC was used as the precious-metal free alternative to Pt/XC; C45 was used due to its excellent electrical conductivity as a carbon additive in LIBs; and SFG was used as a crystalline alternative to the amorphous carbon for improved 82 lithiation capacities.211 Table 2.2 summarizes the physicochemical properties of the carbon-based components used in the RL of the cathode GDE. The sprayed ink also includes IPA and DI H2O (1:0.7 vol/vol) as the solvent and Nafion ionomer as the binder (5 wt%, Sigma-Aldrich). In some cases, N-methyl-2-pyrrolidone (NMP) and polyvinylidene fluoride (PVDF) were used as the alternative solvent and binder, respectively. The final ionomer content was kept at 30 wt% in the RL regardless of the recipe. The crystallinity of the graphitic SFG and Pt/XC was confirmed by X-ray diffraction (XRD, Bruker D2 Phaser) as shown in Fig. 2.8. The electrode before and after the LNRR tests was characterized with the scanning electron microscope (SEM, Hitachi S-2600 Variable Pressure) and energy dispersive X-ray analysis (EDX). Table 2.2 Physicochemical Properties of the Carbon-based Reaction Layer in the Cathode GDE. C-NERGY SUPER C45 C-NERGY SFG6L Vulcan XC72R 40 wt% Pt/XC Manufacturer Imerys Imerys Cabot Sainergy BET (Brunner-Emmett-Teller) surface area, m2 g−1 45211 17211 234212 100213 Average particle size 48 nm211a 3.6 µm211b 50 nm214 3 nm213c Electrical resistivity (@ density), S cm−1 6.2 (@3.1 g cm−3)211d 117 (@3.1 g cm−3)211d N/Ae N/Ae a Surface mean diameter. b Median (i.e., 50th percentile). c Pt particles only. d LIB cathode prepared from LiCoO2 dry-mixed with 3 wt% C45 or SFG6L. e Comparable information not available. 83 2.2.2 Electrolyte Preparation THF without butylated hydroxytoluene (BHT) stabilizer was purchased from Sigma-Aldrich. EtOH (HPLC grade, anhydrous, ≤ 0.003% H2O) was supplied from Sigma-Aldrich. The as-received THF and EtOH bottles were opened in an Ar-filled glove box and dried over molecular sieves (pore size = 4 Å) for over 1 week. The molecular sieves (Sigma-Aldrich) were regenerated in a muffle furnace at 300 °C for 12 h under convective air flow and naturally cooled down to 60 °C before transferring to the glove box for long-term storage. LiBF4 (≥ 98%, anhydrous) was purchased from Sigma-Aldrich and dried at 90 °C under vacuum overnight before use. 1 M LiBF4 was dissolved in THF with 0 – 1.0 vol% addition of EtOH to prepare 25 mL electrolyte solutions. 5 mL molecular sieve was added to the prepared electrolyte for 24 h and each batch was consumed within 3 – 4 days. For each test, 6.5 – 7 mL electrolyte was filtered through a PTFE chromatography syringe filter (0.20 µm pore, 13 mm diameter) to remove trace amounts of impurities for the electrolyte and debris from the molecular sieve. After the filtration, a clear Fig. 2.8 X-ray diffraction results for different carbon-based materials used for the cathode reaction layer. 84 transparent electrolyte was obtained and was carried out of the glove box using an air-tight glass syringe (25 mL, Hamilton), which was connected via Luer lock to a three-way valve as the seal. 2.2.3 Electrolyzer Setup and Operation As shown in Fig. 2.9, the electrolyzer cell is custom-made from polyether ether ketone (PEEK) and consists of a liquid chamber (no baffles) and gas manifold plates for the cathode and anode sides. The single serpentine flow field has square channels of 0.85 mm in width and depth spaced by 0.65 mm landing ribs. The cell was assembled outside of the glove box with the hot-pressed electrodes and bolt-tightened to a fixed torque of 30 in-lb (3.4 Nm) for each screw. Then the electrolyzer cell was connected to the setup shown in Fig. 2.10 and Fig. 2.11 in a fume hood with Swagelok fittings. For the gas supply, N2 and H2 (purity = 99.999%) gases were humidified at room temperature in THF over molecular sieve that was prepared fresh daily in the glove box. Prior to electrolyte injection, 30 sccm N2 was used to purge the cell for 30 min. The exhaust gas from both the cathode and anode sides was sent to an acid absorber (5 mL 0.1 M HCl) to capture possible NH3 in the gas phase generated from the LNRR. The acid absorber outlet was connected to a water column to impose 15-mbar back pressure, which was sufficient to prevent any electrolyte crossover and leakage in the PEEK cell. The exhaust gas was always bubbled through the water column to prevent liquid back flow and possible contamination from the water vapor. A syringe pump (Harvard Apparatus) was used to inject the electrolyte at a rate of 0.5 mL min−1 to a target volume of 5.25 mL. For periodical sampling in long-term experiments, 1-mL electrolyte was collected from the outlet of the liquid chamber and refilled with an aliquot fresh electrolyte. Two control experiments were conducted: (1) open circuit potential with N2-saturated electrolyte (N2 as the cathode gas supply); and (2) electrolysis with Ar-saturated electrolyte (Ar as the cathode gas supply). 85 Fig. 2.9 Electrolyzer cell designs for the LNRR studies in non-aqueous electrolyte. A: cross-section view of a flow cell electrolyzer that incorporates GDEs for the LNRR on the cathode and the HOR on the anode. B: exploded view of the flow cell electrolyte used for this study. The GDE structure is detailed in the inert. Fig. 2.10 Schematic of the flow cell electrolyzer. The PEEK cell refers to the design in Fig. 2.9. 86 2.2.4 LNRR Experiments with the Flow Cell Electrolyzer The LNRR experiments were performed in a three-electrode configuration connected to a Biologic potentiostat (VMP-3, Biologic Science Instruments). For the cathode (CA), GDEs spray-coated with various carbon-based electrocatalysts were examined, whereas a Pt/XC GDE alone was used as the anode (AN) to facilitate the HOR. A gold wire (0.25 mm in diameter, 99.95% purity from Alfa Aesar) was encapsulated in PEEK tubing and inserted into the liquid chamber as the pseudo-reference electrode (RE). The assembly of the PEEK bolted cell is shown in Fig. 2.12. After electrolyte injection, the OCP was measured for 10 min with N2 flowing to both the cathode and anode. Then the anode gas supply was switched to H2 for another 10-min at OCP. Then Fig. 2.11 Image of the flow cell electrolyzer. The electrolyzer was set up in the fume hood out of the glove box. 87 chronopotentiometry (CP) electrolysis was conducted with a constant (or cycled) current density between −1 to −5 mA cm−2 for 4 h, or until the potentiostat was overloaded (|𝐸𝑐𝑒𝑙𝑙| > 10 V). For the cycled current density operation, 1-min CP at desired current density was performed followed by 1-min OCP. Completing the 4-h electrolysis required 8 h for the overall operation duration. Before and after the electrolysis, EIS was performed at OCP with a magnitude of 10 mV. The solution resistance (𝑅𝑠) taken from the intercept with the real axis in the Nyquest plot at the beginning of the test was used for the manual compensation of ohmic losses. After the LNRR test, all cell components (syringe, plates, seals, humidifiers, acid absorbers, and valves) were cleaned with IPA and then DI H2O thoroughly and dried at 55 °C in an oven under vacuum overnight. Fig. 2.12 Images of the assembly of the flow cell electrolyzer. The order follows: bottom 316 stainless steel bearing plate (A), anode flow field plate (B), M4 316 stainless steel screws (C), anode gas-diffusion electrode (D), liquid chamber plate (E), cathode gas-diffusion electrode (F), cathode flow field plate (G), and top 316 stainless steel bearing plate (H). Note that 5/16-inch holes were punched out of the gas-diffusion electrodes to avoid electrical contact with the screws. The assembled cell (H) was connected to the electrolyzer setup as shown in Fig. 2.11. 88 The THF solvent was previously reported to experience electrochemical oxidation that leads to a ring-opening polymerization process in non-aqueous environments, as described in Fig. 2.13.215 The electrolyte color change accompanying the anodic electrolyte oxidation was measured by UV-Vis spectroscopy using a 7-mL glass vial over the wavelength range between 200 – 900 nm. To verify the oxidative THF polymerization on the anode, the viscosity of selected samples was evaluated by generating the flow curves using a modular compact rotational rheometer (MCR 302e, Anton Paar). Rotational rheometry was performed using concentric cylinders with 17-mm or 100-mm inner diameter for low- and high-viscosity samples, respectively. The zero-shear viscosity is reported as the average shear viscosity from 100 s−1 to 20 s−1. Fig. 2.13 Anodic THF oxidation and the subsequent cationic polymerization.215 89 Table 2.3 lists the experimental parameters for the LNRR experiments conducted in the flow cell electrolyzer. The above electrolyzer design and control parameters (i.e., gas types, LiBF4 concentration and gas type) were optimized during the early stages of this project, and are described in detail in Appendix B Electrolyzer Prototype for the Non-aqueous LNRR and Optimization of Operating Conditions. Particularly, preliminary experiments show that employing H2 as the anode gas and/or adding EtOH to the electrolyte can effectively prevent the THF degradation (coloration and/or oxidative polymerization), due to the H+-donating functionality. Table 2.3 Experimental Conditions for the LNRR Experiments with the Flow Cell Electrolyzer. Experimental Variables Reaction layer component Cathode: Pt/XC, Vulcan XC72R, C45, SFG; Anode: Pt/XC Binder in the reaction layer Nafion (IPA/H2O as solvent); PVDF (NMP as solvent) Current density (𝑗, mA cm−2) −1, −3, −5 Electrolysis mode Constant 𝑗 or cycled 𝑗 (1-min electrolysis followed by 1-min OCP) EtOH (vol%) 0, 0.5, 1.0 2.2.5 Half-cell Experiments in a Single-compartment Glass Cell To calibrate the pseudo-RE and inspect the role of EtOH in Li deposition, a glass batch cell (BASi Corp.) analogous to Fig. 2.14A was used in the Ar-filled glove box. A Pt wire, a Pt coil (BASi Corp.) and an Au coil were used as the WE, CE and pseudo-RE, respectively (Fig. 2.14B). For the 90 calibration of the Au pseudo-RE, Li was plated under CP for 5 min at 𝑗 = −5 mA cm−2 in 1 M LiBF4 without EtOH, followed by holding at the OCP for 1 min. Then a CV scan between −0.5 to 0.3 V versus the OCP was conducted at 20 mV s−1 for three cycles. As demonstrated in Fig. 2.15, the OCP after Li deposition and the intercept between the CV scan and the 𝑥-axis both show a consistent value of 𝐸𝐿𝑖+/𝐿𝑖 = −3.42 VAu. Fig. 2.14 A: cross-section view of a commonly used liquid-flooded single-compartment cell where the CA, AN, and RE are all submerged in the liquid electrolyte. B: the glass single-compartment cell, which was used in the Ar-filled glove box. Fig. 2.15 Calibration of the pseudo-Au reference electrode versus the Li+/Li equilibrium potential (ELi+/Li = −3.04 VRHE). A: chronopotentiometric (CP) Li plating in 1 M LiBF4 electrolyte and the following open circuit potential (OCP). B: cyclic voltammetry (CV) scan near the OCP. 91 For the Li deposition experiment, CV was performed in 6.5 mL 1 M LiBF4 electrolyte with or without 0.5 vol% EtOH using freshly cleaned Pt wire at a series of scan rates (1 – 500 mV s−1) within a non-faradaic range (−0.1 and −0.2 VAu). The peak currents (𝑖𝑐/𝑖𝑎) for the anodic and cathodic scan were measured against the voltage scan rate (𝑣 ) to calculate the double-layer capacitance (𝐶𝑑𝑙 =𝑖𝑎 (𝑜𝑟 −𝑖𝑐)𝑣) of the WE (Fig. 2.16). Based on the geometric surface area (𝐴𝑔𝑒𝑜 = 0.788 cm2) of the submerge section of the WE, the area specific capacitance (𝐶𝑑𝑙̅̅ ̅̅ =𝐶𝑑𝑙𝐴𝑔𝑒𝑜) can be calculated and compared for different electrolyte compositions. Then CV scans near the Li deposition region (−2 – −4 VAu) were performed for 10 cycles, and the charges for the Li deposition (𝑄𝑑𝑒𝑝𝑜𝑠𝑖𝑡) and stripping (𝑄𝑠𝑡𝑟𝑖𝑝) for each cycle were integrated to calculate the charge efficiency (𝜂 =𝑄𝑠𝑡𝑟𝑖𝑝𝑄𝑑𝑒𝑝𝑜𝑠𝑖𝑡). Lastly, Li was deposited on the same Pt wire using CP at 𝑗 = −5 mA cm−2 for 30 min, followed by holding at OCP for 1 h. After the above series of tests, the Pt wire was polished with 2500-grit abrasive paper followed by flame-annealing with a blow torch. The rest electrodes Fig. 2.16 Typical CV results to measure the double layer capacitance (Cdl) of the Pt wire electrode in different electrolyte compositions in the glass cell. A: CV scans at varying scan rates within a non-faradaic region. B: anodic and cathodic peak currents at different scan rates with linear fit results. The experiments were conducted in the Ar-filled glove box. 92 and glass cell were sonicated for 30 min each in IPA and DI H2O, followed by drying in the vacuum oven at 90 °C overnight. All area normalization references the 𝐴𝑔𝑒𝑜 of the working electrode (0.788 cm−2 for Pt wire in the glass cell and 4.62 cm2 for the flow cell). 2.3 Analytical Methods 2.3.1 Materials and Chemicals The chemicals for NH4+ quantification, sodium pentacyanonitrosylferrate(III) dihydrate (Na2[Fe(CN)5NO]·2H2O, ≥ 99%), salicylic acid (C7H6O3, ≥ 99%), citric acid anhydrous (C6H8O7, ≥ 99.5%), sodium hypochlorite (NaOCl, 11 – 15% available chlorine), sodium hydroxide (NaOH, ≥ 97%), and ammonia sulfate ((NH4)2SO4, ≥ 99%) were supplied by Sigma-Aldrich Corp. For the hydrazine (N2H4) quantification, 4-dimethylaminobenzaldehyde ((CH3)2NC6H4CHO, ≥ 99%), hydrochloric acid (HCl, 37%), ethanol (C2H5OH, ≥ 96%), and hydrazine monohydrate (N2H4·H2O, 64−65% available N2H4) were also purchased from Sigma-Aldrich Corp. Polystyrene cuvettes (Thermo Fisher Scientific Inc.) with a 10-mm pathlength were used for spectrophotometry measurements within the visible light range. 2.3.2 NH3 Quantification in Aqueous Solutions The NH4+ concentration was measured using the indophenol blue method adapted from the Berthelot reaction (equation 2.3): (2.3) Each sample was tested for three times and the standard deviation was calculated as the error bar. Briefly, 2 mL of electrolyte sample was mixed with 0.2 mL sodium nitroprusside (1 wt%) in a polystyrene cuvette, followed by adding 0.5 mL of 4 M NaOH aqueous solution that also contains 5 wt% salicylic acid and 5 wt% sodium citrate. Shortly after that (within 1 min), 0.5 mL sodium 93 hypochlorite (1.1 − 1.5% available chlorine) was added. The mixture was then capped with polyethylene lids and incubated in the dark for 2 h before analyzing with the UV-Vis spectrophotometer (GENESYS 20TM, Thermo Fisher Scientific Inc.). The peak absorbance at 675 nm was used to establish a calibration curve to calculate the NH4+ content in the sample solution (Fig. 2.17). The total ammonia nitrogen (i.e., NH3 and NH4+) was reported in the unit of parts per million (ppm). Moreover, a lower detection limit of 0.05 ppm was assigned due to the departure Fig. 2.17 Calibration for the measurement of NH4+ based on the indophenol blue method. A – B: The light absorbance spectra between 550 – 800 nm for NH4+ standards within 0.00 – 0.10 and 0.0 – 1.0 ppm range, respectively. C – D: The calibration curves based on the peak absorbance at 675 nm for NH4+ standards within 0.00 – 0.10 and 0.0 – 1.0 ppm range, respectively. The minimum detection limit was set as 0.05 ppm to prevent outliers. 94 from linearity in the calibration curve in ranges below this threshold. Finally, for NH4+ levels below 0.1 ppm, the linear regression parameters differ from those within the 0.1 – 1.0 ppm range. Therefore, two sets of linear fitting were used for these two regions to calculate the NH4+ concentration precisely. 2.3.3 NH3 Quantification in Non-aqueous Solutions After the electrochemical characterization, the electrolyte samples were collected for the dry-redissolution procedure to remove the significant amounts of THF. The acid absorber also captured THF vapor from the THF-humidified gas exhaust and therefore acid samples also required dry-redissolution. The cathode and anode electrodes were dried at ambient conditions overnight and soaked in 5 mL HCl (0.1 M) for at least 24 h. As shown in Table 2.4, a series of calibration standards were prepared using the blank electrolyte consistent to the electrolyte used for the electrolysis (1 M LiBF4 in THF with 0 – 1 vol% EtOH). DI H2O and 20-ppm NH3 stock solution, diluted from 1000-ppm NH4Cl aqueous solution, were added to the electrolyte calibration standards in 7-mL borosilicate glass vials to make known 𝑐𝑁𝐻3 between 0 – 10 ppm with 2-ppm increments. To fixate the volatile NH3 that is potentially present in the electrolyte, 0.1 mL HCl (4 M) was added to the electrolyte samples as well as the non-aqueous standards. Meanwhile, 5 mL acid samples were transferred to the glass vials without additional acidification. The above non-aqueous samples (calibration standards, electrolyte, acid) were heated in dry bath (a hot plate with an embedded heating block) at 70 °C for 24 – 48 h until completely evaporated. After cooling to room temperature, 2 mL DI H2O or HCl (0.1 M) were added to the dried samples according to Table 2.4 and well mixed. In addition, a series of aqueous calibration standards with known 𝑐𝑁𝐻3 between 0 – 10 ppm were prepared. 95 Table 2.4 Preparation of Non-aqueous and Aqueous Samples for the IB Measurement Process Sample Volume (mL) Sample 4 M HCl 20-ppm NH3 DI H2O Redissolve (DI H2O) Redissolve (0.1 M HCl) Dry-redissolution Calibration-1 (0 ppm) 0.5 0.1 0 1 2 Calibration-2 (2 ppm) 0.5 0.1 0.2 0.8 2 Calibration-3 (4 ppm) 0.5 0.1 0.4 0.6 2 Calibration-4 (6 ppm) 0.5 0.1 0.6 0.4 2 Calibration-5 (8 ppm) 0.5 0.1 0.8 0.2 2 Calibration-6 (10 ppm) 0.5 0.1 1 0 2 Electrolyte 0.5 0.1 0 1 2 Acid 5 2 Aqueous indophenol blue method Calibration-1 (0 ppm) 1 0 0 1 Calibration-2 (0.1 ppm) 1 0 0.01 0.99 Calibration-3 (0.2 ppm) 1 0 0.02 0.98 Calibration-4 (0.5 ppm) 1 0 0.05 0.95 Calibration-5 (0.8 ppm) 1 0 0.08 0.92 Calibration-6 (1 ppm) 1 0 0.1 0.9 Calibration-7 (2 ppm) 1 0 0.2 0.8 Calibration-8 (3 ppm) 1 0 0.3 0.7 Calibration-9 (4 ppm) 1 0 0.4 0.6 Calibration-10 (5 ppm) 1 0 0.5 0.5 Calibration-11 (6 ppm) 1 0 0.6 0.4 Calibration-12 (7 ppm) 1 0 0.7 0.3 Calibration-13 (8 ppm) 1 0 0.8 0.2 Calibration-14 (9 ppm) 1 0 0.9 0.1 Calibration-15 (10 ppm) 1 0 1 0 Redissolved samples 2 Cathode soaking solution 2 Anode soaking solution 2 96 The IB method was then performed for the redissolved samples, aqueous calibration standards, and electrode soaking solutions. The same coloring reagents were added to glass vials following the same sequence as in Section 2.3.2. Gradually, a blue color develops in samples that contain NH3 within 10 min. As debris exists from the drying process, the IB samples were filtered using 0.22-µm PTFE filters until transparent. The filtrate was then incubated in dark for 2 h before analysing with the UV-Vis spectrophotometer (GENESYS 20TM, Thermo Fisher Scientific Inc.). The peak absorbance at 660 nm for the aqueous calibration standards was used to calculate the 𝑐𝑁𝐻3 in the acid and electrode samples. Meanwhile, the peak absorbance at 670 nm for the non-aqueous electrolyte calibration standards was used to calculate the 𝑐𝑁𝐻3 for the electrolyte samples. As shown in Fig. 2.18C, the peak wavelength for the non-aqueous samples shifted from 690 to 660 nm between 0.5 h and 2.0 h after preparation with improved linearity between absorbance and 𝑐𝑁𝐻3. Therefore, results from the 2 h case was chosen to calculate the 𝑐𝑁𝐻3 for both the aqueous and non-aqueous samples. 97 Fig. 2.18 Typical IB calibration for the aqueous and non-aqueous samples. A: the full spectrum of the aqueous calibration standards collected at 0.5 h and 2.0 h after preparation. B: the peak absorbance at 660 nm of aqueous calibration standards and linear fits at 0.5 h (R2 = 0.9899) and 2.0 h (R2 = 0.9948). C the full spectrum of the non-aqueous calibration standards collected at 0.5 h and 2.0 h after preparation. D: the peak absorbance at 670 nm and 690 nm of non-aqueous calibration standards and linear fits at 0.5 h (R2 = 0.9589) and 2.0 h (R2 = 0.9946), respectively. The peak shifted in value between 0.5 h and 2.0 h for the non-aqueous electrolyte with an increased R2. Therefore, results from 2.0 h were used for the NH3 measurement. 98 2.3.4 Additional Analytical Methods The Watt-Chrisp method was employed to analyze N2H4 concentration (in ppm NO3−-N) in the electrolyte sample as a possible NH4+ oxidation product/intermediate.13 The coloring reagent was prepared by dissolving 5.99 g para-dimethylamino-benzaldehyde, and 30 mL concentrated HCl in ethanol for a total volume of 500 mL. 2 mL of electrolyte sample was mixed with 2 mL of coloring reagent homogeneously and the mixture was measured by the spectrophotometer after 10 min incubation time. The peak absorbance at 460 nm was used for the calibration curve (Fig. 2.19). For the NO3− measurement (in ppm NO3−-N) as the contaminant in freshly prepared MEAs or as another possible product from NH4+ oxidation, the electrolyte sample was spectrophotometrically measured within the ultraviolet range (190−300 nm) using a quartz cuvette (Starna Cells Inc.). The peak absorbance at 205 nm was measured to establish the calibration curve (Fig. 2.20). To estimate the NO2− concentration (in ppm NO2−-N), a coloring reagent was obtained by dissolving 52.5 mL concentrated HCl, 2.5 g sulfanilamide, 0.25 g N-(1-naphthyl)ethylenediamine dihydrochloride, and 68 g sodium acetate in H2O for a final volume of 250 mL. 2 mL of electrolyte sample was added to 2 mL coloring reagent and the mixture was analyzed 10 min after the mixing with the UV-Vis spectrophotometer within the range of 390 – 650 nm. The peak absorbance at 505 nm was used to build the calibration curve (Fig. 2.21). 99 Fig. 2.19 Calibration for the measurement of N2H4 based on the Watt-Chrisp method. A: The light absorbance spectra collected between 400 – 550 nm for the N2H4 standards. B: The calibration curve based on the peak absorbance at 460 nm of N2H4 standards within the 0.1 – 1.0 ppm N2H4-N range. Fig. 2.20 Calibration for the measurement of NO3− by the direct spectrophotometry scanning between 200 – 225 nm. A: The light absorbance spectra for the NO3− standards. B: The calibration curve based on the peak absorbance at 205 nm of NO3− standards within the 0.0 – 1.0 ppm NO3−-N range. 100 Generally, only small quantities of N2H4, NO2− and NO3− (< 0.1 ppm) were detected from the electrolyte (Fig. 2.22), which indicates insignificant NH3 oxidation in NH4+ crossover experiments. This demonstrates that it is the NH4+ transfer instead of reaction that leads to the detected 𝑐𝑁𝐻4+ changes. The lack of NH3 oxidation activity might be due to the low pH condition in the anode compartment and/or the suppression by Na+ ions as described in previous literature.216,217 Fig. 2.21 Calibration for the measurement of NO2− by the colorimetry technique. A: The light absorbance spectra for the NO2− standards scanned between 390 – 650 nm. B: The calibration curve based on the peak absorbance at 505 nm of NO2− standards within the 0.0 – 1.0 ppm NO2−-N range. Fig. 2.22 Typical UV-Vis spectra for the analyses of N-containing compounds in the electrolyte as possible NH4+ oxidation intermediate or reaction before and after the electrolysis test. A: N2H4, B: NO2−, and C: NO3−. Electrolysis conditions: c0CA= c0AN = 100 ppm NH3-N, j = −10 A m−2. 101 2.4 Performance Metrics Calculation and Unit Conversion For the non-aqueous ENRR tests, the NH3 production rate (𝑟𝑁𝐻3, μmol h−1 cm−2) was calculated as: 𝑟𝑁𝐻3 =∑ 𝑐𝑁𝐻3𝑖 × 𝑉𝑖𝑖𝑀𝑊 × 𝐴𝑔𝑒𝑜 × 𝑡 (2.1) where 𝑐𝑁𝐻3𝑖 is the NH3 concentration in ppm of the electrolyte, acid absorber, and electrode soaking solutions measured by the IB method; and 𝑉𝑖 is the corresponding solution volume in mL. 𝑀𝑊 is the molecular weight of NH3 (17 g mol−1), 𝐴𝑔𝑒𝑜 is the geometric surface area of the electrode (5 cm2), and 𝑡 is the electrolysis duration in h. The faradaic efficiency (𝐹𝐸, %) was calculated as: 𝐹𝐸 =∑ 𝑐𝑁𝐻3𝑖 × 𝑉𝑖𝑖 × 𝑛 × 𝐹𝑀𝑊 × 𝐴𝑔𝑒𝑜 × 𝑡 × 𝑄 × 106× 100% (2.2) where 𝑛 is the number of transferred e− per NH3 molecules (𝑛 = 3), 𝐹 is the faraday constant (96485 C mol−1), and 𝑄 is the charge that passes during the electrolysis test in Coulombs. In the literature, various units have been used for 𝑟𝑁𝐻3 and the conversion factors are listed in Table 2.5. Table 2.5 Calculation and Conversion Factors for Different Units for rNH3 in the Literature. Unit Calculation Conversion factor μmol h−1 cm−2 N/A 1 μg h−1 cm−2 × 𝑀𝑊𝑔𝑚𝑜𝑙 17 nmol s−1 cm−2 ×103𝑛𝑚𝑜𝑙𝜇𝑚𝑜𝑙÷ 3600𝑠ℎ 0.278 Occasionally, the 𝑟𝑁𝐻3 was not explicitly calculated in the literature but the current density (𝑗, mA cm−2) and 𝐹𝐸 are available to calculate the corresponding 𝑟𝑁𝐻3 in μmol h−1 cm−2: 102 𝑟𝑁𝐻3 =𝑗 × 𝐹𝐸𝑛𝐹× 103𝜇𝑚𝑜𝑙𝑚𝑚𝑜𝑙× 3600𝑠ℎ (2.3) This methodology was used to calculate and verify published results as summarized in Fig. 1.2, Fig. 1.3 and Table 5.1. 103 Chapter 3 Identification of False Positives in the Aqueous ENRR 3.1 Introduction For the baseline evaluation, the ENRR performance of a freshly-prepared Au/KB or MoS2 gas diffusion electrode (GDE) was assessed in the TP5E electrolyzer in a neutral 0.1 M Na2SO4 aqueous solution. A gas-liquid co-flow configuration was used only for the cathode compartment, whereas the anode side only flows humidified H2 in single-pass (Fig. 2.7A). Despite clear increases in the current density for the linear scan voltammetry (LSV) with N2 compared to Ar, the ENRR electrolysis at constant 𝐸𝑐𝑒𝑙𝑙 does not produce significant 𝑐𝑁𝐻3 that surpasses the background nitrogenous contaminants. The adventitious NH3 detected during the LSV tests and control experiments clearly demonstrated the false positives collected from the ENRR evaluation of Au/KB and MoS2 GDEs. Additionally, it is postulated that NH3/NH4+ crossover with the Nafion separator might facilitate the NH3/NH4+ transfer during the ENRR baseline collection and control experiments. This inspired further investigations/verifications of the NH4+ crossover with different separator materials reported in Chapter 4. 3.2 Results and Discussion 3.2.1 Au/KB GDE as the cathode As shown in Fig. 3.1A, the LSV scans are compared for the Au/KB GDE with Ar or N2 co-flow to the cathode compartment with the 0.1 M Na2SO4 electrolyte. Pt/XC deposited onto the Nafion N112 membrane was used as the anode catalyst-coated membrane (CCM). 10 scans were performed for the LSV test under each gas atmosphere and the 10th cycle were selected for the comparison. The current density of the N2 LSV is more negative than the Ar case consistently in the region between −0.3 – −0.8 V. The increase in current density indicates the occurrence of additional reduction reaction(s) other than the background HER, which is often attributed to the 104 ENRR in the literature despite the possibility of the nitrogenous compounds (e.g., NO3− and NO2−) undergoing reduction.25 To verify the ENRR capability of the Au/KB GDE, the same MEA was assessed with potentiostatic polarization for 4 h under N2 at an 𝐸𝑐𝑒𝑙𝑙 within the range of −0.3 – −0.8 V, as suggested by the LSV comparison to exhibit active ENRR. Fig. 3.1B shows the 𝑗 – 𝑡 curve for the electrolysis as well as the following Ar control experiment. At 𝐸𝑐𝑒𝑙𝑙 > −0.4 V, the 𝑗 increases gradually over the 4-h electrolysis for both N2- or Ar-saturated conditions. Such MEA activation could be explained by the surface hydrophilicity increases of the cathode carbon paper substrate (GDS3215) that contains a PTFE-treated microporous carbon layer, which has been reported previously for structurally analogous carbon papers.218 As the GDE becomes more hydrophilic, the electrolyte breakthrough pressure reduces, which allows for improved ionic contact between the electrode and the liquid electrolyte to enhance the 𝑗 during the electrolysis. After the LSV and ENRR experiments, the electrolyte (catholyte) and the acid absorber were analyzed for their NH3 content. As summarized in Fig. 3.2A and Fig. C.11, NH3 was detected Fig. 3.1 A: LSV comparison (10th cycle) for the Au/KB GDE under Ar or N2 atmosphere. B: j – t curve for the ENRR experiments with N2 and control test with Ar. 105 from LSV experiments under N2 and Ar atmosphere with a decreasing trend with respect to the test order. The same gas-switch LSV comparison was repeated, and the detected NH3 continues the diminishing trend to reach below the blank background (ca. 1 μg). For the ENRR experiment at different 𝐸𝑐𝑒𝑙𝑙 (Fig. 3.2B), the low NH3 contents detected are comparable to the Ar control and blank background. Additionally, the N2-saturated OCV control test demonstrates much higher NH3 (6.4 μg) than the electrolysis case. Considering the high background NH3 detected from the initial LSV and OCV control experiments, it is likely that negligible ENRR performance could be achieved with the Au/KB GDE. The surprisingly high 𝑐𝑁𝐻3 from the OCV test was confirmed by repeating the same set of repetitive LSV scans and ENRR electrolysis with another MEA of the same electrode materials in Error! Reference source not found.2. Considering the absence of recirculated electrolyte to collect and examine the NH3 content during the LSV nor ENRR tests, it is possible that the anode compartment might collect NH3 that crosses from the cathode side during the electrolysis. During the OCV test, the residual NH3 on the anode Fig. 3.2 NH3 contents in the electrolyte and acid absorber from the LSV and ENRR experiments with the Au/KB GDE. A: repeated LSV with gas switch between N2 and Ar for twice. B: ENRR tests at different Ecell with N2 and control experiments with OCV and Ar comparison. NH3 in the blank electrolyte and acid solution was labelled as the reference line. The shaded parts represent control experiments. 106 side might transfer back to the cathode side and become captured. As a result, the 𝑐𝑁𝐻3 detected at OCV is consistently higher than the ENRR baseline collection. By using a symmetric flow configuration (Fig. 2.7B), the hypothesis of NH3/NH4+ crossover with the Nafion membrane was examined and confirmed based on results found in Chapter 4. 3.2.2 MoS2 GDE as the cathode As an alternative to the commercially available Au/KB as the electrocatalyst of the cathode GDE, MoS2 was deposited (loading = 0.5 mg cm−2) onto the carbon paper substrate using a hydrothermal method.192 The resulting MoS2 GDE was tested for the ENRR performance following the same procedure as the Au/KB case. The initial LSV characterization (Fig. 3.3A) shows an increase of ca. 0.2 mA cm−2 in current density when switching the gas supply from Ar to N2, indicating possible ENRR activities. Continuous potentiostatic electrolysis was then conducted between 0.05 – −0.4 V in cell voltage for 4 h along with control experiments at OCV in N2-saturated electrolyte) and electrolyte in Ar-saturated electrolyte (Fig. 3.3B). Fig. 3.3 A: LSV comparison (10th cycle) for the MoS2 GDE under Ar or N2 atmosphere. B: j – t curve for the ENRR experiments with N2 and control test with Ar. 107 As shown in Fig. 3.4, the NH3 contents in the electrolyte and in acid absorbers were analyzed. Like the Au/KB GDE case, NH3 was also detected from the electrolyte/acid absorbers for the initial LSV test (N2-saturated), followed by a significant reduction for the next LSV test (Ar-saturated). Unlike Au/KB GDE, however, ca. 3 µg NH3 was obtained from ENRR experiments conducted at low 𝐸𝑐𝑒𝑙𝑙 = –0.05 and –0.10 V, which exceeds the NH3 detected from the control experiments. Further elevating the 𝐸𝑐𝑒𝑙𝑙 above –0.10 V leads to diminished NH3 production, resulting in similar NH3 contents as the control results. The positive ENRR results at the low 𝐸𝑐𝑒𝑙𝑙 range also corresponds to a low current density of ca. –0.02 – –0.05 mA cm–2. According to Wu et al.219, although both N2 and H adatom could co-adsorb onto the modelled heterogenous electrocatalytic sites (Ru-N4 single-atom catalysts), the following potential-dependent protonation step favors the Heyrovsky process to generate H2 (i.e., the HER) over the formation of adsorbed *NNH intermediate toward NH3 formation (i.e., the ENRR following associative reaction mechanism). Additionally, under elevated electrode potentials, the kinetic barrier reduces more significantly for Fig. 3.4 NH3 contents in the electrolyte and acid absorber from the LSV and ENRR experiments with the MoS2 GDE. NH3 in the blank electrolyte and acid solution was labelled as the reference line. The shaded parts represent control experiments. 108 the HER than the ENRR, therefore leading to a lower selectivity toward the NH3 formation. Hence, the higher NH3 production at a lower 𝐸𝑐𝑒𝑙𝑙 range could be explained by the absence of the competing HER at the same electrode potential. To verify the presence of ENRR with the MoS2 GDE, long-term electrolysis experiments were conducted up to 24 h. To prevent excessive rates of HER, the electrolysis was conducted under constant current densities (i.e., galvanostatic polarization), which also showed more stable signals than the constant voltage case. As summarized in Fig. 3.5A – B, two separate cells were respectively tested within two current density ranges to evaluate the dependence of ENRR performance on the current density level: (1) low current density range: –0.01 – –0.10 mA cm–2 Fig. 3.5 Long-term ENRR electrolysis with the MoS2 GDE at two different current density ranges. A – B: cell voltages for the electrolysis tests at a current density of –0.01 – –0.10 mA cm–2 and –0.04 – –1.00 mA cm–2, respectively. C – D: NH3 concentration in the electrolyte over the electrolysis duration. 109 and (2) high current density range: –0.04 – –1.00 mA cm–2. Among those continuous electrolysis tests, linear increases of 𝑐𝑁𝐻3 were consistently observed for –0.04 mA cm–2 from two individual GDEs over at least 20 h (Fig. 3.5C – D). The highest 𝑐𝑁𝐻3 of 0.14 ± 0.01 ppm and 0.09 ± 0.01 ppm were obtained at 16 h and 20 h, respectively, for the two individual repeats. The NH3 yield, however, decreases significantly with increased or decreased current densities, possibly due to higher levels of competing HER and insufficient kinetic energy to active the N2 protonation in response to the respective current density changes.25,219 In terms of the ENRR performance, the NH3 production rates (𝑟𝑔𝑒𝑜) and Faradaic efficiencies (𝐹𝐸) were calculated throughout the long-term electrolysis tests. For the low current density range (Fig. 3.6A – B), the 𝑟𝑔𝑒𝑜 and 𝐹𝐸 for the –0.01 mA cm–2 case reached as much as 1.3 ± 0.1 x 10–12 mol s–1 cm–2 and 3.7 ± 0.3%, respectively, during the initial electrolysis (0 – 2 h). However, the performance diminished quickly to less than 0.2 x 10–12 mol s–1 cm–2 and 0.5% for those performance matrices. At –0.04 mA cm–2, the MoS2 GDE seems to best maintain its ENRR activity and selectivity. As shown in Fig. 3.6C – D, ca. 1 – 2 x 10–12 mol s–1 cm–2 in 𝑟𝑔𝑒𝑜 and 0.8 – 1.4% in 𝐹𝐸 was achieved over the 24-h period despite a moderate decreasing trend. The diminished ENRR performance over extended electrolysis periods (> 2 h) was previously reported by Du et al.45 using a MoS2-based electrocatalyst, which was attributed to the self-poisoning of active sites by the produced NH3. Additionally, while ab initio DFT calculations have theoretically predicted Mo or MoS2 as likely active sites that can efficiently bind with N2 toward its activation,220 those investigations hardly consider the applied solvent and NH3 solvation in it. Therefore, the NH3 desorption could become the rate-limiting step when the vicinity of the active sites is saturated with the produced NH3.45 Nevertheless, considering the presence of a N-110 containing precursor, thiourea, for the synthesis of MoS2, it is possible that the detected NH3 could originate from background nitrogenous compounds. The release of contaminants could inevitably lead to the artificially high ENRR performances in Fig. 3.6, particularly during the initial electrolysis (0 – 2 h). With a low current density (–0.01 mA cm–2), the erroneous results could be amplified by a low overall coulombic charge, which ultimately resulted in the highest 𝐹𝐸 in Fig. 3.6C. Regardless of the origin of the observed performance degradation, or even its validity, conducting NH3 electrosynthesis at the optimized –0.04 mA cm–2 will not be practical given a low 𝑐𝑁𝐻3 (< 0.2 Fig. 3.6 NH3 production rates (rgeo) and Faradaic efficiencies (FE) for the MoS2 GDE in two sets of long-term electrolysis experiments at constant current densities. A – B: rgeo for the cell tested within –0.01 – –0.10 mA cm–2 and –0.04 – –1.00 mA cm–2 ranges, respectively. C – D: FE for the ENRR characterization with MoS2 GDEs. 111 ppm) and 𝑟𝑔𝑒𝑜 (1 – 2 x 10–12 mol s–1 cm–2). At this NH3 production level, the background contamination, especially for the initial performance characterization, could always potentially mask the genuine aqueous ENRR results. 3.3 Conclusion In conclusion, this chapter evaluated the ENRR performance of freshly-prepared Au/KB and MoS2 cathode GDEs in a flow-cell electrolyzer following the common practice in the literature. Despite increased current densities for the LSV scans with N2 compared to Ar, the ENRR electrolysis at constant cell voltages or current densities does not yield significant NH3 beyond 0.2 ppm. Control experiments were also conducted, which unfortunately indicates false positives from background contaminants. While it is possible that NH3 was genuinely produced with the MoS2 cathode GDE at low applied electrode potentials, performance degradation and low operating current densities not only led to questions for false positives, but also suggested impracticality of applying aqueous ENRR technologies toward industrial-relevant applications. Lastly, the NH3 detected from initial LSV tests and OCV control experiments inspired for further investigation on the origin of adventitious nitrogenous contaminants in the flow cell electrolyzer and discovery of apparent NH3/NH4+ crossover with different separator materials even at 1-ppm levels in Chapter 4. 112 Chapter 4 Interactions between NH3/NH4+ and Separator Materials in the Flow Cell Electrolyzer 4.1 Introduction From the electrolyzer design perspective, guidelines and an in-depth understanding of how different cell components cooperatively improve or compromise the ENRR performance have yet to receive sufficient attention. Both the H-cell with stand-alone electrodes and flow electrolysis cells with membrane electrode assemblies (MEAs) have been used to assess the performance of electrocatalysts in previous ENRR studies, with the H-cell being the predominant choice due to its structural simplicity.38,40,53,221 The flow cell design, where GDEs can be integrated to transport gaseous N2, offers additional opportunities to effectively alleviate the mass transfer constraints of N2 caused by its marginal solubility (0.6 mM in H2O) and poor diffusivity (2 x 10−5 cm2 s−1 in H2O) at ambient conditions.38,222,223 For both categories of cell layouts, a separator is usually used and expected to compartmentalize the catholyte and anolyte (Fig. 4.1A) so that the possible NH3 (or ammonium, NH4+, in neutral or acidic aqueous electrolytes) produced by the ENRR cathode does not move to the anode and become oxidized.199 For these purposes, the Nafion membrane is often used as the separator and solid electrolyte. Nevertheless, the effectiveness of Nafion in Fig. 4.1 Schematics of electrochemical cells with a CEM as the separator for A: NH4+ electrosynthesis and B – C: NH4+ removal for wastewater treatment with cathode- or anode-fed operations, respectively. 113 preventing NH4+ crossover remains debatable.46,198 In addition, NH4+ can potentially adsorb in Nafion membranes (or ionomers if used in catalyst layers as the binder) due to the Brønsted acid character of sulfonate groups (–RSO3−) and therefore, accumulate adventitious NH3/NH4+ from the surroundings or genuine products in ENRR experiments.27,44 As a consequence, in the case of NH4+ crossover and/or adsorption in Nafion, inaccurate conclusions could arise in assessing the electrocatalyst performance or identifying false positives in the ENRR studies. For example, Ren et al.46 show that approximately 30 % of 2 µg mL−1 (equiv. part per million or ppm) NH4+ added to acidic (HCl), neutral (Na2SO4), and alkaline (KOH) catholyte solutions in an H-cell can cross Nafion membranes of different thicknesses within 2 h. The uptake of NH4+ by the separator in dilute NH4+ (0.1 µg mL−1) solutions was corroborated with evidence obtained from NH3 temperature-programmed desorption, Fourier transform infrared spectrum, and elemental analysis techniques. The NH4+ adsorption in Nafion membranes was also supported by Hanifpour et al.44 as they advocate for proper pre-treatment procedures to be strictly followed before ENRR tests. Moreover, apart from reporting the NH4+ movement across Nafion N117 membranes in both an H-cell and a flow electrolysis cell, Lv et al.199 also unveiled that using HCl as the anolyte can expedite the crossover processes because the anodic oxidation of HCl generates Cl2 which hydrolyzes by H2O into hypochlorous acid (HClO). The resulted HClO rapidly oxidizes any crossover NH4+ from the catholyte into N2 (2𝑁𝐻4+ + 3𝐻𝐶𝑙𝑂 → 𝑁2 + 5𝐻+ + 3𝐶𝑙− + 3𝐻2𝑂 ). Conversely, Cai et al.198 demonstrated satisfactory capabilities of Nafion N211 membranes in preventing any NH4+ crossover with negligible NH4+ adsorption in an H-cell. Unfortunately, such decent NH4+ separating abilities were not verified under electrolysis conditions, which might influence the NH4 crossover or absorption behavior by factors such as Nafion property change or cation migration.210,224 114 In the flow cell configuration, on the other hand, the interaction between NH4+ and the Nafion membrane has been rarely discussed for NH4+ electrosynthesis applications. Considering the significance of flow electrolysis cells in incorporating GDEs for improved N2 mass transfer and scaling up the ENRR process, the compatibility of Nafion membranes need to be scrutinized and understood. As one of the few examples, Chen et al.221 indicated that the NH4+ generated on the cathode GDE can easily cross the Nafion N115 membrane, and adding a bare carbon paper between the membrane and the GDE contributes to a higher NH4+ production rate, which might be due to restricting the NH4+ crossover to the anode chamber. Unfortunately, the authors did not quantify the crossed NH4+ nor the membrane's susceptibility to NH4+ adsorption in the flow cell. In the fields of H2 and NH3 fuel cells, however, the crossover of gaseous or dissolved NH3 through Nafion membranes has been recognized, and it impacts the electrochemical performance of the fuel cell. Traces of NH3 (5−200 ppm) mixed in H2 gas feed can lower the H2 fuel cell performance by reducing the ionic conductivity of the solid electrolyte and/or passivating the electrocatalytic activity of the anode and cathode catalyst layers (although no NH3 is present in the oxygen feedstock).225,226 The so-called solution-diffusion mechanism originating from the ion exchange between H+ and NH4+ at the Nafion hydrophilic –RSO3− moieties was proposed to enable the NH3 crossover.227,228 For direct ammonia fuel cells, the cell voltage typically drops well below the thermodynamic equilibrium potential (1.17 V with O2 as the oxidant) due to the NH3 crossover through the AEM separator, thus diminishing the power density.229,230 Although NH3 crossover has been widely observed in DAFCs, NH3 concentrations are generally high (≥ 0.1 M) and crossover behaviors with low NH3 contents (1 ppm) have not been investigated. From a NH4+ remediation perspective, electrochemical systems using the CEM (Fig. 4.1B) are proposed as an energy-benign and cost-effective option to replace the anaerobic oxidation of 115 NH4+.231 Generally, natural or synthetic wastewater containing appreciable amounts of NH4+ (e.g., 0.1 – 10 g/L or equiv. 100 – 105 ppm NH4+) is fed to the anode side and crosses to the catholyte due to concentration gradient (Δ𝑐𝑁𝐻4+) and electric field (𝜙).231,232 As the cathode reaction (i.e., HER) consumes H+ or generates OH− via the HER, the catholyte pH increases to pass the acid dissociation constant of NH4+ (𝑝𝐾𝑎 = 9.25 at 25 °C), which results in significant amounts of deprotonated NH3 in the catholyte that can be collected by pervaporation membrane or aeration. While there are structural similarities in the electrochemical cell design, the treatment of NH4+ permeation across the CEM differs significantly in NH4+ electrosynthesis vs. NH4+ removal/recovery applications. In the former, NH4+ crossover is unfavourable, but its severity often lacks emphasis. In contrast, in the latter application selective NH4+ ion transport is required for achieving high NH4+ removal efficiencies. Typically, the NH4+ removal is usually performed by feeding NH4+ to the anode side to leverage the migration of NH4+ cations (Fig. 4.1B). However, Xie and Popat233 reported higher NH4+ removal (83.5 – 99.6% within 1 h at −30 A m−2) with the cathode-fed operation (Fig. 4.1C) than the anode-fed counterpart (72 ± 2%) with Nafion N115 as the separator, contradicting the expected superior performance of the anode-fed NH4+ removal. Therefore, further research is needed to investigate the influence of feed location on the NH4+ crossover in remediation cells. Furthermore, Rodrigues et al.224 showed experimentally that the ratio between current density (𝑗) and 𝑐𝑁𝐻4+ needs to be maintained sufficiently high to achieve NH4+ removal efficiencies above 90% in the anode-fed operation. The same authors also showed in another modelling work that further increasing 𝑗 beyond certain limiting current region lowers the percentage of NH4+ transported to the catholyte due to the preferential migration of other cations present in the electrolyte (e.g., H+ and Na+).234 Kuntke et al.235 also observed mixed ion transport across the CEM 116 with fluxes proportional to the respective ion concentration ratios. For Nafion N117, Rozendal et al.236 reported a higher selectivity for Na+ than NH4+ when the molar concentration ratio of the two is high (ca. 4). The practical significance of selecting appropriate separators to prevent product losses during NH4+ electrosynthesis will be even more crucial in the future when NH4+ yields from aqueous ENRR could exceed the current laboratory scale (≤ 1 ppm) by orders of magnitude.29 Recent work by Wang et al.204 reported a high NH4+ yield using polyoxometalate as the redox-mediator in a flow cell to efficiently activate N2 and the synthesized 𝑐𝑁𝐻4+ reached 61.0 ppm. The Nafion N115 used in the study as the separator was found to allow NH4+ crossover and impair product collection, which demonstrates the importance of selecting an appropriate separator. Validating (or disproving) those hypotheses will be of greater practical significance in selecting appropriate separators to prevent product losses during NH4+ electrosynthesis operations. Such insights will perceivably become more crucial when future NH4+ yield from aqueous ENRR shall be lifted from the current laboratory scale (≤1 ppm) by orders of magnitude with rational electrocatalyst/electrolyzer designs and modular scale-up.29 A recent work by Wang et al.204 reported a high NH4+ yield using polyoxometalate as the redox-mediator in a flow cell to efficiently activate N2 and the synthesized 𝑐𝑁𝐻4+ accumulated substantially up to 61.0 ppm. Nafion N115 used in the study as the separator was found to allow NH4+ crossover and impair product collection, which demonstrates the importance of selecting appropriate separator for the accurate performance assessment toward the ENRR and effective product collection. Herein, NH4+ crossover and removal/recovery were evaluated in a flow cell electrolyzer with Nafion 112 membrane in relation to the operating conditions (𝑗, 𝑐𝑁𝐻4+ , and feed location). It is 117 demonstrated that Nafion does not function as an inert nor effective barrier against NH4+ crossover in electrolyzers with a flow cell configuration, even at the 1-ppm level. Furthermore, the N112 membrane’s inability to retain NH4+ within the electrolyte recirculation loop is a result of different factors, including crossover, volatilization, and adsorption. Hence, distinct relocation sites are unveiled in different electrolyte solutions, including the opposite electrolyte compartment, the acid trap, and/or the membrane. Particularly, the transient behaviour of the electrolyte pH is highlighted as the main cause of inconsistent NH4+ crossover in a neutral electrolyte under different cell operating conditions. Lastly, while the ion exchange with Na+ was shown as an effective cleaning procedures to remove adsorbed NH4+ from the Nafion membrane and/or ionomer, the electrolysis conditioning could also be used to ensure both membrane-bound nitrogenous contaminants and H+ are sufficiently removed to avoid interference against 𝑐𝑁𝐻4+ measurement and interpretation of NH4+ crossover. Alternative separators were also examined for their reliability toward NH3 electrosynthesis: thicker CEM (N350), AEM (Sustainion X37-50), and microporous diaphragms (Celgard C3400, C3500, and C5550). 118 4.2 1-ppm NH4+ Crossover with Nafion N112 in Different Electrolyte Compositions 4.2.1 NH4+ Crossover through Nafion N112 in H2SO4 Fig. 4.2 shows the NH4+ concentration changes in the catholyte and anolyte in acidic media. The cathode-fed case simulates the situation when NH4+ is produced from genuine ENRR (or fictious nitrogenous contaminants) on the cathode, whereas the anode- and symmetric fed operations are considered to verify the direction of NH4+ crossover. At OCV (Fig. 4.2A), NH4+ crosses the MEA following the Δ𝑐𝑁𝐻4+ between the two electrolyte compartments (Fig. 4.2C). For the cathode-fed operation, 𝑐𝐶𝐴 drops by 0.35 ppm, which is mirrored by a simultaneous 0.36 ppm increase in the anolyte. Comparable concentration changes were found for the anode-fed mode, which suggests anolyte-to-catholyte NH4+ movement following a reversed concentration gradient. When no Fig. 4.2 NH4+ crossover in 0.05 M H2SO4 in the flow cell electrolyzer using Nafion N112 as the separator. A: OCV; B: GP at −0.1 mA cm−2; C: schematic of NH4+ crossover following the ΔcNH4+. The filled symbols and solid lines represent the catholyte results, and the empty symbols and dashed lines show the anolyte results. 119 Δ𝑐𝑁𝐻4+ was provided in the symmetric fed case, NH4+ transport across N112 is insignificant (< 0.1 ppm). At −0.1 mA cm−2 (equiv. −1 A m−2), NH4+ crossover induced by the Δ𝑐𝑁𝐻4+ was also observed and applying a small 𝑗 shows minor influences on the NH4+ crossover (Fig. 4.2B). In the cathode-fed case, a 0.35 ppm decrease was found for the 𝑐𝐶𝐴 whereas a concurrent increase of 0.34 ppm was detected in the anolyte, which overlaps with the OCV results. However, for the anode-fed mode, over half (0.58 ppm) of the initial 𝑐𝐴𝑁 was transferred to the catholyte, which exceeds the crossed NH4+ at OCV. Considering the cation migration including NH4+ towards the cathode, it is plausible that the enhanced NH4+ crossover to catholyte is assisted by the applied 𝑗 along with the Δ𝑐𝑁𝐻4+ .237 4.2.2 NH4+ Volatilization through Nafion N112 in NaOH In 0.1 M NaOH, the electrolyte pH = 13 exceeds the NH4+ 𝑝𝐾𝑎 (9.25 at 20 °C), and NH4+ almost completely deprotonates into the dissolved NH3 (99.98 mol%). The dissolved NH3 (or referred to as ammonia water) has high vapor pressure and can be rapidly entrained from the liquid phase by gases (Ar and H2) that continuously pass through the electrolyte during the experiment. NH4+ volatilization is termed to describe the total ammonia-nitrogen (NH3 and NH4+) losses in alkaline media to avoid confusion in distinguishing the speciation between NH3 and NH4+. For the cathode-fed mode, only 0.21 ppm NH4+ remains in the catholyte after 7 h at OCV (Fig. 4.3A). Comparable trends can be observed for the anode- and symmetric fed operation where NH4+ volatilization is also obvious. The reduction in the NH4+ concentration corresponds to the NH3 evaporation as the entrained NH3 was captured by the acid absorbers (Fig. 4.4). Compared with the escaped NH3 from the acidic electrolyte (Fig. 4.4A), one order of magnitude higher NH4+ was 120 detected in the acid absorbers (primarily in the first one) when the alkaline solution was used (Fig. 4.4B). Besides, the symmetric fed operation resulted in a roughly two-fold increase in the captured NH4+ in contrast to the cathode- or anode-fed case, which can be explained by the simultaneous NH4+ evaporation losses from both the catholyte and anolyte. Unlike acidic media where a symmetric gain can be found in the opposite electrode compartment, merely 0.08 ppm of the initially added NH4+ in the catholyte crosses to the anolyte at 𝑡 = 7 h. Despite a small proportion (≤ 10%), the crossover of volatilized NH4+ suggests that NH3 diffusion can not only happen to AEMs in DAFCs but also to CEMs in ENRR electrolyzers using alkaline media.228,229 In Fig. 4.3B, rapid NH4+ losses also occur at −0.1 mA cm−2 with minor NH4+ crossover to the blank electrolyte Fig. 4.3 NH4+ crossover in 0.1 M NaOH in the flow cell electrolyzer using Nafion N112 as the separator. A: OCV; B: GP at −0.1 mA cm−2; C: schematic of NH4+ volatilization with minor crossover following the ΔcNH4+. The filled symbols and solid lines represent the catholyte results, and the empty symbols and dashed lines show the anolyte results. 121 side. The effect from the applied 𝑗 on NH4+ crossover cannot be assessed given such a small degree of NH4+ crossover. 4.2.3 NH4+ Adsorption and Desorption in Nafion N112 in DI H2O Next, without any supporting electrolyte, the behaviour of NH4+ in DI H2O were explored (Fig. 4.5A – B). At both OCV and GP, the NH4+ concentration diminishes significantly to below 20% Fig. 4.4 NH4+ captured by the acid absorbers in the exhaust from both the cathode and anode. 6 and 25 mL 0.5 M H2SO4 were added to the first and second acid absorbers, respectively. Nafion 112 was used as the separator. A – D: H2SO4, NaOH, H2O, and Na2SO4 in the electrolyte, respectively. Note the break in y-axis in B. The NH4+ concentration configurations (1 or 0 ppm) and the test condition (OCV or GP) are labelled in the horizontal labels. The horizontal reference lines represent the blank. 122 of the initially added amount and no recovery of such loss was observed in the opposite electrolyte compartment. Although the volatilization was the primary cause for the NH4+ loss from NaOH, it does not completely account for the sharp drop of NH4+ in DI H2O. While Fig. 4.4C shows apparent NH4+ in the acid absorbers after the OCV and GP tests, the captured NH4+ is far less than the NH4+ removed from DI H2O, resulted in net NH4+ losses for the overall mass changes that consider both the liquid electrolyte and acid absorbers (Fig. 4.6C). Fig. 4.5 NH4+ adsorption in and desorption from the N112 membrane. A – B: NH4+ crossover tests at OCV and GP (j = −0.1 mA cm−2) in DI H2O, respectively. The filled symbols and solid lines represent the catholyte results, and the empty symbols and dashed lines show the anolyte results. C: NH4+ desorption to blank 0.1 M Na2SO4 from a contaminated MEA membrane used for crossover tests. D: Decreasing amounts of NH4+ were released from consecutive desorption tests in blank 0.1 M Na2SO4. 123 Several hypotheses could explain the diminished NH4+: (1) NH3 vaporization or electrolyte leakage in the recirculation loop; (2) the anodic decomposition of NH3 or NH4+ to N2 or NOx−;47,217 and/or (3) the retention of NH4+ in the MEA. The first hypothesis could be ruled out by repeating the crossover test using polyimide films (Kapton) as the separator, which show good retainment of NH4+ within each electrode compartment (Fig. D.1). Secondly, anodic oxidation is unlikely because no net electron transfer occurs at OCV. Besides, neither the detected NH4+ in the anolyte nor the 𝑗 at GP is sufficiently high for significant consumption of NH4+. Hence, the ion exchange Fig. 4.6 The NH4+ mass gain or loss in the catholyte, anolyte, and acid absorbers (bars) and the overall mass balance (lines and symbols) for the crossover tests under OCV and GP conditions. A – D: crossover results in H2SO4, NaOH, H2O, and Na2SO4 as the liquid electrolyte, respectively. The NH4+ concentration configurations (1 or 0 ppm) and the test condition (OCV or GP) are labelled in the horizontal labels. 124 between NH4+ and H+ in the N112 membrane is the most probable explanation for the depletion of NH4+ from DI H2O. To clarify whether NH4+ from H2O was retained in the membrane, the solution was replaced with blank 0.1 M Na2SO4 (no NH4+) after performing the NH4+ crossover tests in DI H2O. As shown in Fig. 4.5C and Error! Reference source not found.2, significant NH4+ desorption was observed at OCV from the MEA that was used for crossover tests in DI H2O, likely due to the ion exchange between Na+ and NH4+ cations in the Nafion.209,238 The accumulated large quantities of NH4+ in Nafion can be rapidly released to the blank Na2SO4 in a short period of time (ca. 5 min) with 23.9 and 37.9 μg NH4+ found in the catholyte and anolyte, respectively. The released NH4+ continues to increase rapidly for the first hour followed by a plateauing trend. The desorbed NH4+ eventually accumulates to 92.9 and 118 μg at 7 h in the catholyte and anolyte, respectively, for the initial OCV test. The NH4+ desorption test was repeated in blank Na2SO4 for four consecutive times at OCV or GP (𝑗 = −0.1 mA cm−2) conditions (Fig. 4.5D). The desorbed NH4+ shows a monotonic decreasing trend in such repetition until negligible NH4+ was obtained for the final desorption test. Based on mass balance, 260 ± 7 μg NH4+ desorbs from the contaminated membrane, which accounts for nearly 90% of the diminished NH4+ from crossover tests in DI H2O (297 ± 10 μg). As the electrolytic system was always thoroughly rinsed and recirculated with DI H2O (no NH4+ added) after each crossover test, it is unlikely that the desorbed NH4+ into the blank Na2SO4 originates from the remnant electrolyte in previous crossover tests, but instead from the NH4+-imbibed membrane only. Conversely, it could also be inferred that the disappearance of NH4+ from DI H2O in the crossover test originates from the ion exchange between NH4+ and H+ at the –RSO3− functional groups in Nafion.227 For further proof, NH4+-containing DI H2O and blank Na2SO4 (0.1 M) solutions were added to the opposite sides of a fresh MEA to repeat the NH4+ crossover tests. 125 Fig. 4.7A and Fig. 4.7B shows that NH4+ consistently moves from the DI H2O side to the Na2SO4 side at both OCV and GP conditions, regardless of the polarity (cathode or anode) of the MEA. Therefore, the NH4+ adsorption from DI H2O into Nafion membrane can occur simultaneously with the NH4+ desorption into blank Na2SO4 following the ion exchange principle as depicted in Fig. 4.7C. 4.2.4 NH4+ Crossover though Nafion N112 in Na2SO4 The NH4+ crossover was then examined in neutral Na2SO4 electrolyte. In general, the NH4+ crossover was observed at both OCV and GP conditions and its occurrence depends on the NH4+ fed location. At OCV (Fig. 4.8A), NH4+ was well separated by the N112 membrane for the anode-fed operation, but nearly all the NH4+ in the catholyte moves to the anolyte for the cathode- and Fig. 4.7 NH4+ crossover tests with NH4+-containing DI H2O and blank Na2SO4 in separated electrolyte compartments using Nafion N112 as the separator. A: OCV; B: GP at −0.1 mA cm−2; C: schematic of NH4+ adsorption and desorption in DI H2O and in Na2SO4. The filled symbols and solid lines represent the catholyte results, and the empty symbols and dashed lines show the anolyte results. 126 symmetric modes. Similar results were observed for the GP condition (Fig. 4.8B) where the NH4+ separation was found only in the anode-fed case. Additionally, for scenarios when the NH4+ Fig. 4.8 1-ppm NH4+ crossover in 0.1 M Na2SO4 (ca. pH 6) using Nafion N112 as the separator. The MEA was conditioned initially in blank 2 M Na2SO4 followed by equilibrating in blank 0.1 M Na2SO4. A – B: NH4+ concentration changes (ΔcNH4+) at OCV and GP, respectively. C – D: electrolyte pH changes (ΔpH) at OCV and GP, respectively. The filled symbols and solid lines represent the catholyte results, and the empty symbols and dashed lines show the anolyte results. E: schematic of NH4+ crossover through Nafion N112 membrane in Na2SO4. The catholyte and anolyte pH increases and decreased due to the HER and the HOE, respectively. E 127 crossover was observed (i.e., cathode- and symmetric fed), the 𝑐𝐶𝐴 losses are comparable at OCV and GP, whereas the 𝑐𝐴𝑁 gains at GP is ca. 0.2 ppm lower than the counterparts at OCV. As the NH4+ crossover still occurs even with no initial Δ𝑐𝑁𝐻4+ (for the symmetric fed operation) or after Δ𝑐𝑁𝐻4+ drops to zero (for the cathode-fed operation), the observed NH4+ movement is unlikely induced by diffusion alone. Rather, the anolyte tends to accumulate NH4+ which prevents the equilibrium between the 𝑐𝐶𝐴 and 𝑐𝐴𝑁 . According to the observations in acid and alkaline electrolyte media, the electrolyte pH was found to influence the NH4+ distribution. Therefore, the electrolyte pH for the crossover test in Na2SO4 was measured to correlate with the NH4+ crossover (or separation). As shown in Fig. 4.8C and Fig. 4.8D, similar trends in pH changes were found for the OCV and GP conditions where the catholyte (anolyte) becomes more alkaline (acidic). Similar to the crossover results in alkaline, an increased pH in catholyte can lead to NH4+ deprotonation and hence NH3 diffusion through the Nafion separator (Fig. 4.8E). Meanwhile, the anolyte captures the diffused NH3 with a lower pH and therefore results in the NH4+ crossover toward the anolyte. Likewise, for the anode-fed operation, the acidified anolyte can retain NH4+ effectively since any diffused NH4+ to the catholyte due to Δ𝑐𝑁𝐻4+ is likely to become volatilized and spontaneously diffuse back (in the NH3 form) to the anolyte.239 As described in Section 2.1.6, the Nafion membrane was converted to the Na+-form while the H+ conductivity was reduced significantly.208,209 Therefore, the H+ consumed (generated) at the cathode (anode) during the electrolysis in Na2SO4 can not be balanced, which results in the observed pH polarization in Fig. 4.8D. Unlike GP, the reason for the electrolyte pH changes at OCV is less obvious. Without 𝑗, H+ is not electrochemically consumed (or generated) at scale and does not impact the electrolyte pH significantly. Instead, the ion exchange between the liquid-128 bound and membrane-bound Na+ and H+ might induce the electrolyte pH polarization, especially when the ionic strength of the Na2SO4 solution is changed. Specifically, between crossover tests, DI H2O was recirculated overnight to clean residual NH4+ from the electrolytic system, which essentially makes the MEA to equilibrate with DI H2O and partially release the membrane-bound Na+. Hence, when the membrane was exposed to 0.1 M Na2SO4 solution again for the following crossover test at OCV, the re-adsorption of solution-bound Na+ enables the desorption of membrane-bound H+. The dynamic-state effect of Na+ in a CEM H2O electrolyzer was described by Zhang et al.240, where the cation poisoning (i.e., ion exchange) in the MEA was found to be partially reversible when switching the liquid environment between 1 mM Na+ solution and DI H2O. However, it is challenging to ascribe the causes for the preferential H+ accumulation in the anolyte given asymmetric electrode configurations (GDE and CCM) and gas types (Ar and H2) in the current system. Applying microscopic (e.g., electron probe microanalyzer240 and pH microprobe241), spectroscopic (e.g., infrared 242) tools to monitor the distribution of Na+ and local pH in the MEA might offer further insights. The correlation between the NH4+ crossover in Na2SO4 and electrolyte pH changes can be also reflected by the NH4+ detected in acid absorbers. In Fig. 4.4D, NH4+ was found in cases of NH4+ crossover (i.e., cathode- and symmetric fed operations), which indicates NH4+ volatilization that allows for NH3 to evaporate with the exhaust gas. Conversely, negligible NH4+ was detected in the acid absorbers for the anode-fed operation. Hence, most of the evaporated NH3 originates from the catholyte that becomes more alkaline, but to a less extent from the acidified anolyte. Such hypothesis is further supported by the observation that the captured NH4+ in the acid absorbers for the cathode-fed operation is comparable to the symmetric-fed case in Na2SO4. Otherwise, a significant increase in the acid-trapped NH4+ should be expected when NH4+ was added to both 129 the cathode and anode compartments, similar to the NaOH case (Fig. 4.4C). Additionally, higher amounts of NH4+ in the acid absorbers were detected at GP than OCV, which corresponds to a slight drop in the crossed NH4+ to the anolyte in Fig. 4.8B than Fig. 4.8A. Those results align with the greater pH increases in the catholyte at GP than OCV that enhances the deprotonation of NH4+ and increases the vapor pressure of NH3. As a result, greater NH3 evaporates to the acid absorbers rather than diffusing to the anolyte during the electrolysis operation in comparison to the OCV. 4.2.5 Implications for NH4+ Electrosynthesis in Flow Electrolysis Cells Although the studied concentration gradient (1 ppm) is too low to be relevant in practical NH4+ electrosynthesis, acknowledging the severity of NH4+ permeation through the Nafion membrane remains critical at a laboratory scale. Table 4.1 summarizes the NH4+ crossover flux normalized to the geometric surface area of the N112 membrane (5 cm2) for the cathode-fed operation. Many previously reported NH4+ production rates from N2 in H-cells are below or close to the crossover flux (100 – 300 µmol m−2 h−1 or equiv. 10 – 30 nmol cm−2 h−1) that was observed herein.223 Hence, if the same electrocatalyst for half-cell studies were to be benchmarked in flow cells for full-cell tests, examining potential NH4+ crossover would be imperative for accurate performance assessments. 130 Table 4.1 NH4+ Flux at OCV and GP for the Cathode-fed Operation (ΔcNH4+= 1 ppm) with N112 Membrane.2 Test NH4+ flux from the catholyte to anolyte (µmol m−2 h−1) H2SO4 (0.05 M) NaOH (0.1 M) DI H2O DI H2O and Na2SO4 (0.1 M) Na2SO4 (0.1 M) OCV 116 ± 12 77 ± 14 0.1 ± 5.8 173 ± 6 115 ± 26 GP (−0.1 mA cm−2) 128 ± 8 59 ± 11 0.6 ± 4.6 243 ± 8 247 ± 23 Furthermore, the adsorption of NH4+ in the Nafion was verified for different electrolyte solutions. Following the ion exchange approach to remove the adsorbed NH4+ after crossover tests in DI H2O, blank Na2SO4 (0.1 M) solution was recirculated in the electrolyzer after completing the crossover tests in different types of electrolyte solutions. As shown in Fig. 4.9, the severity of NH4+ contamination in Nafion membranes follows the order: DI H2O >> Na2SO4 > H2SO4 > NaOH. Notably in NaOH, the Nafion membrane experienced the least NH4+ retention, likely due to the high alkalinity of the solution that volatilizes most NH4+ and prevents its adsorption in the Nafion membrane. 2 Calculated based on cumulative mass changes to the anolyte in crossover tests using different MEAs (Section 4.2.1 – 4.2.4). Mass losses to the catholyte due to NH4+ adsorption and volatilization are not accounted for. 131 Fig. 4.9 NH4+ desorption from contaminated N112 membrane in blank Na2SO4. A – D: desorbed NH4+ from MEAs that were used for crossover tests in H2SO4, NaOH, DI H2O, and Na2SO4 respectively. The filled symbols and solid lines represent the catholyte results, and the empty symbols and dashed lines show the anolyte results. 132 4.3 NH4+ Crossover at Elevated Current Densities and Concentrations 4.3.1 NH4+ Crossover with N112 in Cathode-fed Operation NH4+ crossover for N112 as the separator was first assessed at different 𝑐𝑁𝐻4+ and 𝑗 in the cathode-fed mode. At OCV (Fig. 4.10A), notable NH4+ crossover from the catholyte to the anolyte was observed in the concentration range of 1 to 100 ppm, corresponding to symmetrically decreasing and increasing trends in 𝑐𝐶𝐴 and 𝑐𝐴𝑁, respectively. However, the normalized change in 𝑐𝑁𝐻4+ (i.e., changes to 𝑐𝐶𝐴 or 𝑐𝐴𝑁 normalized to the tested 𝑐𝑁𝐻4+, for example (𝑐𝐶𝐴𝑡 − 𝑐𝐶𝐴𝑡=0)/𝑐𝑁𝐻4+) decreases when the 𝑐𝑁𝐻4+ increases, and negligible NH4+ crossover was witnessed at OCV for the highest 𝑐𝑁𝐻4+ at 0.2 M. In Fig. 4.10B, with a small 𝑗 (−1 A m−2), the NH4+ crossover to the anolyte becomes more pronounced than at OCV for the same 𝑐𝑁𝐻4+. For 1 ppm, 41.3 ± 0.7% NH4+ in the catholyte moves to the anolyte at OCV, which increases nearly two-fold (81.9 ± 4.7%) at −1 A m−2. Like the OCV case, increasing 𝑐𝑁𝐻4+ at −1 A m−2 reduces the normalized NH4+ crossover to the anolyte. Nevertheless, applying −1 A m−2 enables 10.7 ± 0.7% NH4+ crossover for the highest 𝑐𝑁𝐻4+ (0.2 M), which was negligible at OCV. When increasing further the current density to −10 A m−2 and −100 A m−2 (Fig. 4.10C – D), the NH4+ crossover becomes asymmetric. The removed NH4+ from the catholyte was not fully compensated by the recovered NH4+ in the anolyte, and such asymmetric changes are more pronounced for low 𝑐𝑁𝐻4+. Moreover, at the highest 𝑗 of −100 A m−2, similar trends for the 𝑐𝐶𝐴 and 𝑐𝐴𝑁 were observed for 𝑐𝑁𝐻4+ between 1 to 100 ppm, suggesting diminished improvement to the NH4+ removal by the 𝑗 increases. However, for the highest 𝑐𝑁𝐻4+ case (0.2 M), the NH4+ removal continues to improve at −100 A m−2. 133 4.3.2 pH Polarization and Correlation with NH4+ Transport Electrolyte pH was monitored throughout the crossover tests (Fig. 4.11), which shows the critical role pH plays in the NH4+ transport. Due to the HER on the cathode, H+ ions are consumed during GP and, therefore, the catholyte becomes alkaline. Meanwhile, the HOR on the anode acts oppositely and acidifies the anolyte, while H+ is generated. Hence notable pH polarization across Fig. 4.10 Changes to the cCA (filled symbols) and cAN (empty symbols) normalized to the tested cNH4+ for the NH4+ crossover with N112 in the cathode-fed operation. A – D: j = 0 (OCV), −1, −10, −100 A m−2, respectively. Different cNH4+ are color-coded and represented by different symbol shapes. 134 the separator was observed. Fig. 4.11A shows continuous pH increase (decrease) in the catholyte (anolyte) with increased 𝑗 for 1 ppm NH4+. As the partition of NH4+ and NH3 in the electrolyte is directly influenced according to acid-base equilibrium, 𝑁𝐻4+ + 𝑂𝐻− ↔ 𝑁𝐻3 + 𝐻2𝑂 𝑜𝑟 𝑁𝐻4+ ↔ 𝑁𝐻3 + 𝐻+, 𝑝𝐾𝑎 = 9.25 (4.1) most NH4+ is deprotonated into NH3 that evaporates with the aerated gas streams when the catholyte pH increases at −10 and −100 A m−2 well above the 𝑝𝐾𝑎. Fig. 4.11 Electrolyte pH changes with different cNH4+, j, and feed locations for the Nafion N112. A: pH changes during crossover tests for 1 ppm in the cathode-fed operation. B – C: catholyte and anolyte pH at the end of crossover tests, respectively, in the cathode-fed operation. D: electrolyte pH at the end of crossover tests for 3400 ppm with different feed locations. 135 The loss of evaporated NH3 hence leads to the asymmetric concentration changes in Fig. 4.10C – D. The molar flux for the evaporated NH3 from the electrolyte (𝐽𝑁𝐻3𝐸𝑣𝑎𝑝) is estimated as: 𝐽𝑁𝐻3𝐸𝑣𝑎𝑝 =𝑐𝑁𝐻3𝐾𝐻,𝑁𝐻3𝑝𝑡𝑜𝑡 𝐽𝑔𝑎𝑠 (4.2) , where 𝑐𝑁𝐻3 is the NH3 concentration in the electrolyte, 𝐽𝑔𝑎𝑠 is the gas flux that aerates the electrolyte, 𝑝𝑡𝑜𝑡 is the total gas pressure, and 𝐾𝐻,𝑁𝐻3 is the Henry’s constant for NH3. Meanwhile, when the catholyte pH is below or close to the NH4+ 𝑝𝐾𝑎, as seen at OCV and −1 A m−2 in Fig. 4.11A, NH4+ volatilization and subsequent NH3 evaporation is mainly absent. Nevertheless, it is known that the local pH at the electrode-electrolyte interface of the MEA is higher (lower) than that of the bulk catholyte (anolyte).120,243 The locally enhanced pH difference across the Nafion membrane may still enable the deprotonation of NH4+ at the interface into NH3 that diffuses to the anolyte, ultimately resulting in the observed NH4+ crossover even at a low 𝑗 (e.g., Fig. 4.10B). The molar flux of diffused NH3 to anolyte though the separator (𝐽𝑁𝐻3𝐷𝑖𝑓𝑓) follows Fick’s law of diffusion: , where 𝐷𝑒,𝑁𝐻3 is the effective diffusion coefficient of NH3 in Nafion, and 𝑥 is the coordinate normal to the separator plane. For the electrolytic cell under investigation, the relative contribution of 𝐽𝑁𝐻3𝐷𝑖𝑓𝑓 and 𝐽𝑁𝐻3𝐸𝑣𝑎𝑝 was calculated following the work by Liu et al.239, while assuming a constant NH3 concentration gradient throughout the separator length (𝐿): (See Table 4.2 for values of the parameters and constants used for the calculation) 𝐽𝑁𝐻3𝐷𝑖𝑓𝑓 = −𝐷𝑒,𝑁𝐻3𝜕𝑐𝑁𝐻3𝜕𝑥 (4.3) 136 Π =𝐽𝑁𝐻3𝐷𝑖𝑓𝑓𝐽𝑁𝐻3𝐸𝑣𝑎𝑝 =𝐷𝑒,𝑁𝐻3𝐾𝐻,𝑁𝐻3𝑝𝑡𝑜𝑡L𝐽𝑔𝑎𝑠= 3.175 (4.4) When Π ≪ 1 , the effect of NH3 diffusion on the NH4+ removal is considered negligible.239 However, Π > 1 for the current system. Therefore, the NH4+ crossover to the anolyte due to NH3 diffusion cannot be ignored. 137 Table 4.2 System Parameters and Equilibrium Constants to Evaluate the Role of NH3 Diffusion in the Electrolyzer. Constant Equation Value Unit Ref. Acid-base equilibrium Dissociation constant of H2O (𝐾𝑊) 𝐾𝑊 = 𝑐𝐻+𝑐𝑂𝐻− 10−8 mol m−3 244 Dissociation constant of NH4+ (𝐾𝑁𝐻4+) 𝐾𝑁𝐻4+ =𝑐𝐻+𝑐𝑁𝐻3𝑐𝑁𝐻4+ 5.6234 x 10−7 mol m−3 244 Gas-liquid equilibrium Henry’s constant of NH3 (𝐾𝐻,𝑁𝐻3) 𝐾𝐻,𝑁𝐻3 =𝑐𝑁𝐻3𝑝𝑁𝐻3 5.625 x 104 mM bar−1 244 System design parameters Membrane area (𝐴) 5 x 10−4 m2 Catholyte volume (𝑉) 1.5 x 10−5 m3 Anolyte volume (𝑉) 1.5 x 10−5 m3 Membrane thickness (𝐿) 5 x 10−5 m Aeration rate (𝑉𝑔𝑎𝑠) 8.333 x 10−7 m3 s−1 Molar volume (𝑉𝑚) 0.0744 m3 mol−1 Total gas pressure (𝑝𝑡𝑜𝑡) 1 bar Aeration flux (𝐽𝑔𝑎𝑠) 𝐽𝑔𝑎𝑠 =𝑉𝑔𝑎𝑠𝐴𝑉𝑚 4.464 x 10−3 mol m−2 Diffusion coefficients NH4+ in free solution (𝐷𝑓,𝑁𝐻4+) 1.94 x 10−9 m2 s−1 239,245 NH4+ in Nafion (𝐷𝑒,𝑁𝐻4+) 𝐷𝑒,𝑁𝐻4+ = 𝑑𝑓𝐷𝑓,𝑁𝐻4+ 1.94 x 10−10 m2 s−1 239,245 NH3 in free solution (𝐷𝑓,𝑁𝐻3) 2.10 x 10−9 m2 s−1 239,245 NH3 in Nafion (𝐷𝑒,𝑁𝐻3) 𝐷𝑒,𝑁𝐻3 = 𝑑𝑓𝐷𝑓,𝑁𝐻3 2.10 x 10−10 m2 s−1 239,245 Effective diffusion coefficient of NH3/NH4+ in membrane in relation to values in free solution 𝑑𝑓 0.1 239,245 138 Furthermore, the catholyte pH decreases when 𝑐𝑁𝐻4+ increases from 1 to 3400 ppm at the same 𝑗 (Fig. 4.11B), whereas the anolyte pH (Fig. 4.11C) only shows dependence on 𝑗 but not on 𝑐𝑁𝐻4+ . It is likely that the NH4+/NH3 equilibrium buffers the catholyte pH polarization. Therefore, at OCV and −1 A m−2, less NH4+ volatilization and NH3 diffusion losses occur with higher 𝑐𝑁𝐻4+ that reduces the NH4+ crossover in Fig. 4.10A – B. However, as nearly all NH4+ has been deprotonated (Error! Reference source not found.3) for 𝑐𝑁𝐻4+ = 1 – 100 ppm at −100 A m−2 (i.e., 𝑐𝑁𝐻3𝑐𝑁𝐻3+𝑐𝑁𝐻4+ ≈ 1), the NH4+ loss in the catholyte due to 𝐽𝑁𝐻3𝐸𝑣𝑎𝑝 becomes restricted by the 𝐽𝑔𝑎𝑠 , which leads to similar 𝑐𝐶𝐴 decreases in Fig. 4.10D. While the NH3 diffusion is recognized as a major contributor to NH4+ crossover, the NH4+ mass transfer flux (𝐽𝑁𝐻4+) by both diffusion (𝐽𝑁𝐻4+𝐷𝑖𝑓𝑓) and migration (𝐽𝑁𝐻4+𝑀𝑖𝑔) has been frequently considered in previous NH4+ removal/recovery studies, as governed by the Nernst-Planck (NP) equation:239,245 𝐽𝑁𝐻4+ = 𝐽𝑁𝐻4+𝐷𝑖𝑓𝑓 + 𝐽𝑁𝐻4+𝑀𝑖𝑔 = −𝐷𝑒,𝑁𝐻4+ (𝜕𝑐𝑁𝐻4+𝜕𝑥+ 𝑐𝑁𝐻4+𝑒𝑘𝐵𝑇 𝜕𝜙𝜕𝑥) (4.5) , where 𝐷𝑒,𝑁𝐻4+ is the effective NH4+ diffusion coefficient in Nafion, 𝑒 is the electron charge, and 𝑘𝐵𝑇 is the thermal energy. According to eq. (4.5), lower NH4+ crossover to the anolyte is expected with higher 𝑗 due to the opposing electric field effect hampering migration. Such effect could contribute to the observed lower NH4+ crossover to the anolyte in Fig. 4.10 as the 𝑗 increases, but cannot be clearly differentiated from the pH-induced 𝐽𝑁𝐻3𝐸𝑣𝑎𝑝 that depletes NH4+ in the catholyte. Therefore, varying 139 the NH4+ feed location could offer further insight into the role of NH4+ migration as described in the following section. 4.3.3 NH4+ Crossover with N112 in Anode- and Symmetric Fed Operation At OCV and −1 A m−2 in Fig. 4.12A – B, negligible NH4+ crossover for the anode-fed operation was observed for 1 ppm despite the expected NH4+ diffusion to the catholyte. However, such hampered NH4+ crossover to the catholyte agrees with the observed strong NH4+ crossover in the Fig. 4.12 Changes to cCA (filled symbols) and cAN (empty symbols) for NH4+ crossover with N112 in anode-fed operation. A – D: j = 0 (OCV), −1, −10, −100 A m−2, respectively. Different cNH4+ are color-coded and represented by different symbol shapes. 140 opposite (catholyte-to-anolyte) direction for the cathode-fed operation in Fig. 4.10A – B. Therefore, the pH polarization shown in Fig. 4.11A even at low 𝑗 might contribute to the NH4+ transport toward the anolyte (but not the reverse) for 𝑐𝑁𝐻4+ = 1 ppm. For higher 𝑐𝑁𝐻4+ = 10 and 100 ppm, the NH4+ crossover was clearly observed in Fig. 4.12A at OCV. Similar NH4+ removal efficiencies (ca. 30 – 50%) at OCV were reported by Kuntke et al.246 and Desloover et al.247 at comparable operation conditions, which are attributed to the NH4+ diffusion. However, the NH4+ diffusion is negligible for the highest 𝑐𝑁𝐻4+ = 3400 ppm, which agrees with the low 6% NH4+ crossover reported by Rodrigues et al.224 for 𝑐𝑁𝐻4+ = 0.28 M in a continuous anode-fed operation with 1.1 mL min−1 flow rate. One possibility that explains the different 𝐽𝑁𝐻4+𝐷𝑖𝑓𝑓 with different 𝑐𝑁𝐻4+ could be changes in 𝐷𝑒,𝑁𝐻4+ considering a large variation in its value in the literature.239,245 Furthermore, the anode-fed crossover tests also illustrate asymmetric 𝑐𝐶𝐴/𝑐𝐴𝑁 changes with higher 𝑗, which aligns with the prominent pH-induced 𝐽𝑁𝐻3𝐷𝑖𝑓𝑓 for the cathode-fed mode. Besides, the 𝑐𝐶𝐴 increases more rapidly for 𝑐𝑁𝐻4+ = 100 ppm at −10 A m−2 (Fig. 4.12C) than −1 A m−2 (Fig. 4.12B) during the initial 3 h, possibly due to a higher 𝐽𝑁𝐻4+𝑀𝑖𝑔 induced by the higher 𝑗, which was not the case for the cathode-fed operation in Fig. 4.10. 141 The symmetric fed mode, however, emphasizes the NH4+ crossover without the contribution from NH4+ diffusion to reveal the role of NH4+ migration. In agreement with the cathode-fed operation, significant NH4+ crossover to the anolyte with symmetric 𝑐𝐶𝐴/𝑐𝐴𝑁 changes were detected at OCV and −1 A m−2 (Fig. 4.13A – B), and a decreasing trend with higher 𝑐𝑁𝐻4+ . Further increasing 𝑗 leads to not only asymmetric 𝑐𝐶𝐴/𝑐𝐴𝑁 changes due to 𝐽𝑁𝐻3𝐸𝑣𝑎𝑝 losses, but also different crossover directions due to the 𝐽𝑁𝐻4+𝑀𝑖𝑔 toward the catholyte. Specifically, at −10 A m−2 (Fig. 4.13C), NH4+ in Fig. 4.13 Changes to cCA (filled symbols) and cAN (empty symbols) for NH4+ crossover with N112 in symmetric fed operation. A – D: j = 0 (OCV), −1, −10, −100 A m−2, respectively. Different cNH4+ are color-coded and represented by different symbol shapes. 142 the anolyte migrates rapidly to the catholyte for 𝑐𝑁𝐻4+ = 1 – 100 ppm and replenishes the pH-induced NH4+ losses in the catholyte, resulting in slower 𝑐𝐶𝐴 decrease rates than the 𝑐𝐴𝑁. Even initial NH4+ gains for 100 ppm was observed where the catholyte pH is buffered by the NH4+/NH3 equilibrium. However, at −100 A m−2 (Fig. 4.13D), the catholyte pH increase becomes more intense for 1 – 100 ppm (Fig. 4.14C), which leads to significant 𝑐𝐶𝐴 losses due to NH3 evaporation that cannot be compensated by the underlying NH4+ migration from the anolyte. Meanwhile, the pH-induced NH3 diffusion to the anolyte could reach equilibrium with the NH4+ migration to the anolyte, resulting in negligible changes in the 𝑐𝐴𝑁 at −100 A m−2. Similar conclusion was drawn from the modelling work by Dykstra et al.245 that theoretically confirms the competing role between 𝐽𝑁𝐻3𝐷𝑖𝑓𝑓 and 𝐽𝑁𝐻4+𝑀𝑖𝑔 to reduce the anolyte NH4+ removal to only 13% at −20 A m−2. Lastly, as the highest 𝑐𝑁𝐻4+ = 3400 ppm mitigates the electrolyte pH polarization at −100 A m−2 (Error! Reference source not found.5), the 𝐽𝑁𝐻4+𝑀𝑖𝑔 to the catholyte was not balanced by the 𝐽𝑁𝐻3𝐷𝑖𝑓𝑓 to the anolyte, and therefore initial 𝑐𝐶𝐴 gains were observed. The similar results for 100 ppm at −10 A m−2 and 3400 ppm at −100 A m−2 also supports the underlying role of pH polarization in affecting the NH4+ crossover as directly regulated by the 𝑐𝑁𝐻4+ and 𝑗. 143 4.3.4 Recovery Efficiency of NH4+ In addition to removing NH4+ from wastewater, recovering the NH4+ is also essential to the nutrient recycling and environmental sustainability. To assess the flow cell performance toward NH4+ Fig. 4.14 Electrolyte pH changes with different cNH4+, j, and feed locations for the Nafion N112. A – B: catholyte and anolyte pH at the end of crossover tests, respectively, in the anode-fed operation. C – D: catholyte and anolyte pH at the end of crossover tests, respectively, in the symmetric fed operation. 144 recovery, the recovery efficiency (𝑅𝐸𝑁𝐻4+) was calculated based on the captured NH3 in the acid absorbers from the exhaust gas (Error! Reference source not found.4):224 𝑅𝐸𝑁𝐻4+ =∑ 𝑐𝑁𝐻3,𝑎𝑏𝑠𝑖 𝑉𝑁𝐻3,𝑎𝑏𝑠𝑖𝑖=1,2𝑐𝑁𝐻4+𝑉𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑡𝑒× 100% (4.6) , where 𝑐𝑁𝐻3,𝑎𝑏𝑠𝑖 and 𝑉𝑁𝐻3,𝑎𝑏𝑠𝑖 are the captured NH3 concentration and volume in the acid absorbers with 𝑖 representing the 1st and 2nd absorber, respectively. 𝑉𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑡𝑒 is the electrolyte containing NH4+ (15 mL for the cathode-/anode-fed mode, and 30 mL for the symmetric fed mode). In general, Fig. 4.15 shows a decreasing trend in 𝑅𝐸𝑁𝐻4+ with increasing 𝑐𝑁𝐻4+ at the same 𝑗 whereas generally increasing 𝑗 up to −10 A m−2 at a constant 𝑐𝑁𝐻4+ improves the NH4+ recovery. Further increase of the current density without sufficient buffering capacity (i.e., at concentrations of 10 to 100 ppm) is not beneficial for 𝑅𝐸𝑁𝐻4+ because the mass transfer limitation has been achieved.224,234 Meanwhile, the NH4+ crossover was reduced with increases in 𝑅𝐸𝑁𝐻4+ as more 𝐽𝑁𝐻3𝐷𝑖𝑓𝑓 was diverted to 𝐽𝑁𝐻3𝐸𝑣𝑎𝑝 based on eq. (4.4). 145 Furthermore, the anode-fed mode resulted in the lowest 𝑅𝐸𝑁𝐻4+ for 𝑐𝑁𝐻4+ = 1 – 100 ppm in Fig. 4.15A – C because the evaporated NH3 from the catholyte relies mainly on the 𝐽𝑁𝐻4+ transferred from the anolyte, instead of from the initially added NH4+ in the catholyte. However, due to the high buffering capacity from 𝑐𝑁𝐻4+ = 3400 ppm for cathode- and symmetric fed modes (Error! Reference source not found.5), neither the initially added NH4+ to the catholyte nor 𝐽𝑁𝐻4+ from the anolyte is readily volatilized. As the consequence, the anode-fed mode with unbuffered catholyte achieves the highest 𝑅𝐸𝑁𝐻4+ (31.2 ± 0.4%) in Fig. 4.15D rather than the cathode-fed (26.5 ± 2.1%) and symmetric fed (17.4 ± 0.1%) cases. Fig. 4.15 Comparison of NH4+ recovery efficiency using the N112 as the separator at different operation conditions. A – D: 1, 10, 100 ppm, and 0.2 M, respectively. 146 4.4 Importance of MEA Conditioning for Consistent Results The MEA conditioning at −10 A m−2 in blank Na2SO4 electrolyte was found critical in the above presented consistent NH4+ crossover/removal results. Both the nitrogenous contaminants and membrane-bound H+ are two possible factors interfering with the NH4+ behavior. Fig. 4.16A exemplifies the NH4+ contamination in freshly prepared MEA that releases 1.75 and 1.00 ppm NH4+ to the catholyte and anolyte, respectively, during crossover tests at −10 A m−2 for 1 ppm. Consecutive experiments with the same MEA result in reduced background noises as the contaminants were gradually removed. The adventitious NH4+ and NO3− in different MEA components were revealed in Fig. 4.16B, where the homemade Pt/XC-CCM emits the highest NH4+ (17.4 µg) followed by the commercial Au/XC-GDE (11.9 µg). The commercial Pt-CCM (Fuel Cell Etc.) and as-received N112 (without boiling pre-treatment) also release small quantities of NH4+ (3.5 and 1.4 µg, respectively), whereas less than 1 µg NH4+ was found in the homemade Au/KB-GDE and carbon paper substrate (GDS3215, Fuel Cell Etc.). Meanwhile, NO3− was also the fresh N112 shows clear susceptibility to NO3− contamination (1.53 µg). To avoid contamination during MEA storage and handling, the assembled MEA undergoes electrolysis conditioning at −10 A m−2 with blank Na2SO4 prior to crossover tests. As shown in Fig. 4.16C, 2.85 ± 0.78 µg NH4+ was released which was primarily captured by the acid absorbers due to NH4+ volatilization. Repeated conditioning results in less than 0.2 µg NH4+ release with the detected 𝑐𝑁𝐻4+ below the analytical limit (0.05 ppm). 147 Furthermore, the membrane-bound H+ in unconditioned N112 membranes was found to interfere with cation crossover such as vanadium ions,248 as well as the NH4+ crossover in the current system. In Fig. 4.17A, three consecutive crossover tests at −10 A m−2 were conducted for 𝑐𝑁𝐻4+ = 10 ppm. At this 𝑐𝑁𝐻4+ , the initial nitrogenous contaminants should not skew the crossover results as significantly as the 1-ppm case (Fig. 4.16A), and the buffering capacity is insufficient to prevent Fig. 4.16 Effects of nitrogenous contaminants in the MEA with N112. A: fictitious NH4+ released during crossover tests using an unconditioned MEA. B: NH4+ and NO3− contaminants in commercial and homemade (HM) samples. AR: as received. 5 cm2 geometric area was used for each material. C: NH4+ released from the MEA with N112 during electrolysis conditioning at −10 A m−2. D: NH4+ contamination in a fresh MEA (red dash line) versus NH4+ re-introduced from repeated crossover tests at different cNH4+ (black symbol). 148 the catholyte pH polarization above the 𝑝𝐾𝑎 (Fig. 4.11B). However, Test 1 (cathode-fed) and Test 2 (anode-fed) show symmetric 𝑐𝐶𝐴/𝑐𝐴𝑁 changes, contradicting the severe NH3 evaporation for a conditioned MEA (Fig. 4.10C and Fig. 4.12C). Only the symmetric fed operation (Test 3) displays similar trends as the conditioned MEA in Fig. 4.13C. Meanwhile, the 𝑅𝐸𝑁𝐻4+ of Test 1 and Test 2 for the unconditioned MEA is far less than the conditioned counterpart (Fig. 4.18), which also suggests negligible pH-induced 𝐽𝑁𝐻3𝐸𝑣𝑎𝑝. Fig. 4.17 Electrolysis conditioning improves the consistency in assessing the NH4+ crossover for N112 membrane. A: NH4+ crossover results for 10 ppm at −10 A m−2 with the unconditioned MEA. B: catholyte pH during crossover tests for the unconditioned and conditioned MEA. C: Ecell during crossover tests for the unconditioned and conditioned MEA. The chronological order of tests (Test 1 – 3) are labelled for the unconditioned MEA. D: Ecell and catholyte pH for three consecutive electrolysis conditioning at −10 A m−2, which are also labelled chronologically. 149 The catholyte pH changes were tracked in Fig. 4.17B. Unlike the consistent catholyte pH increases for the conditioned MEA, the unconditioned MEA results in lower catholyte pH for Test 1 despite the H+ consumption by the HER at −10 A m−2, which could be attributed to the release of membrane-bound H+.210,242 The catholyte pH increases gradually for Test 2 and Test 3 as the membrane-bound H+ is depleted. The cell voltage (𝐸𝑐𝑒𝑙𝑙) also become more negative in response to different pH polarization results for Test 1 – 3, which eventually reaches the same level as the conditioned MEA (Fig. 4.17C).120,224 By conducting the electrolysis conditioning, both the electrochemical and pH polarization results could be stabilized as shown in Fig. 4.17D, which ensures consistent physicochemical status and crossover results from the MEA. Fig. 4.18 Comparison of NH4+ recovery efficiency for 10 ppm at −10 A m−2 using the N112 membrane that is unconditioned (H+-saturated) or conditioned with electrolysis. 150 4.5 NH4+ Crossover with Alternative Separators In search for separator materials that are suitable for the NH4+ electrosynthesis application, thicker CEM (Nafion N350), AEM (Sustainion X37-50), and microporous diaphragms (Celgard C3400, C3500, and C5550) are evaluated for the NH4+ crossover. Initially, the susceptibility to nitrogenous contaminants of different separators were examined by electrolysis conditioning. Fig. 4.19A – B shows that the highest residual NH4+ of 27.7 ± 2.2 µg was released from X37-50 followed by N350 Fig. 4.19 Electrolysis conditioning at −10 A m−2 for different separator materials. A: NH4+ released to the catholyte, anolyte, and acid absorbers in two separate repeats from MEAs containing different separator materials. B: NH4+ released in three consecutive electrolysis conditioning tests for X37-50. C and D: Ecell for the first and second electrolysis conditioning. Note that for N112, a third electrolysis conditioning (included in Fig. 4.17D) is required 151 (9.2 ± 1.5 µg). Celgard separators show minor contamination (< 5 µg) especially the C3500, which agrees with previous reports.27,31 However, the poor wettability of C3400 and C3500 leads to higher ohmic losses than ion-exchange membranes and hence, much higher 𝐸𝑐𝑒𝑙𝑙 (Fig. 4.19 and Fig. 4.20), whereas N350, X37-50, and C5550 separators provide similar 𝐸𝑐𝑒𝑙𝑙 as the N112. The Fig. 4.20 The poor ionic conductivity of Celgard C3400 and C3500 due to the hydrophobic surface. A: surface contact angle measurement of different Celgard diaphragms. To prevent H2O from permeating into the separator, all images were captured within 30 seconds after the H2O droplet reaches the surface. B: electrolyte pH changes after the crossover experiments. For the symmetric fed operation. C: cell voltages of crossover experiments with Celgard diaphragms. D: impedance measurement for different Celgard diaphragms. 152 surfactant coating on C3400 and C3500 also appears to be removed as reflected by continuously increased 𝐸𝑐𝑒𝑙𝑙.249 As shown in Fig. 4.21A, 1-ppm NH4+ crossover was found at OCV for the cathode-fed operation but less than 40% of the original 𝑐𝑁𝐻4+ in the catholyte moves through the N350 to the anolyte, which reduces significantly from nearly 90% NH4+ crossover through the N112 membrane. Unlike the N112 case, the NH4+ crossover also happens to the anode-fed operation with a comparable trend as the cathode-fed counterpart. However, no NH4+ movement was observed for the symmetric fed test. Those observations largely align with the NH4+ crossover trends in H2SO4 through N112 where the concentration gradient determines the direction of NH4+ movement by inducing the diffusional mass transport. Additionally, negligible pH changes to the electrolyte (Fig. 4.21C) at OCV suggests that an increased thickness can avoid pH perturbation in NH4+-containing electrolyte at near-neutral conditions, which no longer induces the NH4+ volatilization and subsequent NH3 diffusion as seen for the N112. Although further investigation will be required to gain mechanistic insights on the pH variation during OCV operation, the mitigated pH changes with the thicker N350 could be related to an increased number of –RSO3− groups that prevents facile ion-exchange between Na+ and H+ as postulated for the N112 case.250 At −1 A m−2 (Fig. 4.21B), the NH4+ crossover through N350 is evident regardless of the location where NH4+ was initially added. Compared to N112 as the separator, the NH4+ volatilization becomes more severe when the membrane thickness increases. For the cathode-fed operation, the NH4+ gain in the anolyte remains relatively low for the initial 3 h and does not compensate for the NH4+ loss in the catholyte as seen for the N112 in Fig. 4.8B, which could be explained by the lack of NH3 back-diffusion to the anolyte given a thicker membrane separator.251 This observation also 153 coincides with significant increases of NH4+ captured by the acid absorbers (Error! Reference source not found.6). Meanwhile, both the rate of pH changes over the initial 3 h and the overall pH polarization over 7 h are higher for the N350 (Fig. 4.21D) than the N112 separator (Fig. 4.8D), which exacerbates the NH4+ volatilization and the subsequent NH3 evaporation from the catholyte. Similarly, for the anode- and symmetric fed operation, the rapid volatilization of NH4+ in the cathode cannot be Fig. 4.21 1-ppm NH4+ crossover in 0.1 M Na2SO4 (ca. pH 6) using Nafion N350 as the separator. The MEA was conditioned initially in blank 2 M Na2SO4 followed by equilibrating in blank 0.1 M Na2SO4. A – B: NH4+ concentration changes (ΔcNH4+) at OCV and GP (j = −1 A cm−2), respectively. C – D: electrolyte pH changes (ΔpH) at OCV and GP, respectively. The filled symbols and solid lines represent the catholyte results, and the empty symbols and dashed lines show the anolyte results. 154 captured by the acidified anolyte due to the increased membrane thickness. Instead, the NH4+ cation migrates noticeably away from the anolyte under the applied 𝑗 that leads to the significant 𝑐𝑁𝐻4+ drop in the anolyte. Particularly, the initial catholyte NH4+ gain at GP for the symmetric fed case reveals the role of migrational mass transfer under electric field, which is counterbalanced by the continuously increased catholyte pH that results in net NH4+ losses at 7 h. At elevated 𝑗, the NH4+ removal from the catholyte with N350 is limited by the mass transfer of NH3 and shows a maximal 𝑅𝐸𝑁𝐻4+ at −10 A m−2 (Fig. 4.22), which agrees with the trends observed for N112. Similar to the Nafion membrane, the AEM also demonstrates symmetric changes to the 𝑐𝐶𝐴 and 𝑐𝐴𝑁 at OCV and −1 A m−2 in Fig. 4.23A, and higher 𝑗 leads to NH3 evaporation losses. Therefore, despite the oppositely charged functional groups in the AEM, the NH4+ crossover to the anolyte cannot be avoided due to the presence of pH-induced 𝐽𝑁𝐻3𝐷𝑖𝑓𝑓 (Fig. 4.20B). For the Celgard diaphragms (Fig. 4.23B – D), the NH4+ crossover can be mitigated only at OCV with C3400 (6.0 ± 2.3%) and C3500 (10.1 ± 1.1%), which is likely due to the hydrophobic surface Fig. 4.22: RENH4+ for different separator materials in cathode-fed operation under different j. 155 of polypropylene that traps air to fill the micropore voids and prevents the cation movement.27,31,249 With a lower porosity, C3400 shows less NH4+ permeation than C3500, but experiences the disadvantage of a larger ohmic resistance that may become a concern for practical ENRR applications. For C5550, the ohmic loss is much lower due to the hydrophilic fabric layer in addition to the polypropylene layer (Error! Reference source not found.7). However, bulk Fig. 4.23 Changes to the cCA (filled symbols) and cAN (empty symbols) for the NH4+ crossover with AEM and microporous separators at 1 ppm. A – D: j = 0 (OCV), −1, −10, −100 A m−2, respectively. Different feed locations are color-coded and represented by different symbol shapes. For C3400 and C3500, excessive ohmic losses at −100 A m−2 overload the potentiostat (Fig. 4.20) and those results are not shown here. 156 electrolyte crossover was observed due to excessive wetting and pressure gradient across the separator (Error! Reference source not found.8), resulting in asymmetric changes to the 𝑐𝐶𝐴 and 𝑐𝐴𝑁 in Fig. 4.23D. With applied 𝑗, significant NH4+ losses in the catholyte and moderate NH4+ gains in the anolyte were found for all Celgard separators. The bulk electrolyte crossover could introduce inaccuracies to the crossover test with Celgard diaphragms, it remains evident that microporous separators alone cannot perform well in preventing the NH4+ crossover under electrolysis conditions in flow cell electrolyzers. 4.6 Conclusion Herein, in a flow electrolysis cell, the NH4+ separating capability of the Nafion membrane was examined under conditions that are analogous to regular ENRR studies (i.e., low 𝑐𝑁𝐻4+ and low 𝑗). The N112 membrane was first examined in different electrolyte environment: 1. In acidic media, NH4+ crossover occurs substantially following the Δ𝑐𝑁𝐻4+ across the MEA (e.g., from the catholyte to anolyte for the cathode-fed operation). 2. In alkaline media, NH4+ volatilization in the form of NH3 which was captured by the acid absorbers accounts for the majority of depleted NH4+ from the electrolyte solution. Meanwhile, NH3 diffusion was also found following the Δ𝑐𝑁𝐻4+ but represents a smaller percentage than NH3 evaporation. 3. In DI H2O without supporting electrolyte, NH4+ can strongly adsorb in Nafion due to the ion exchange with the membrane-bound H+. The adsorbed NH4+ cannot be removed simply by rinsing with DI H2O but can desorb from the MEA by competitive ion exchange with Na+. 4. In neutral Na2SO4 solutions, the NH4+ crossover (or separation) depends on the operating conditions and the corresponding electrolyte pH changes. The catholyte pH was found to 157 increase consistently at both OCV and GP (𝑗 = −1 A m−2) conditions, whereas the anolyte becomes acidified. Under such pH polarization, NH4+ crossover is only permitted in the catholyte-to-anolyte direction. Besides, elevating the 𝑐𝑁𝐻4+ was found to buffer the pH polarization and mitigates the degree of NH4+ crossover, which validates the correlation between the electrolyte pH and NH4+ crossover behaviours. The NH4+ crossover was then systematically investigated with different separator materials in a wide range of 𝑐𝑁𝐻4+ , 𝑗, and feed location. For the Nafion N112 membrane, NH4+ crossover to the anolyte by up to 81.9 ± 4.7% was observed for 𝑐𝑁𝐻4+ = 1 ppm at −1 A m−2 due to the pH-induced 𝐽𝑁𝐻3𝐷𝑖𝑓𝑓, which reduces to 10.7 ± 0.7% when increasing the 𝑐𝑁𝐻4+ to 3400 ppm. For the same 𝑐𝑁𝐻4+ , increasing the 𝑗 to −10 A m−2 leads to bulk electrolyte pH polarization and diverts the volatilized NH4+ toward 𝐽𝑁𝐻3𝐸𝑣𝑎𝑝 with enhanced 𝑅𝐸𝑁𝐻4+ (78.1 ± 1.1% for 𝑐𝑁𝐻4+ = 1 ppm). The trade-off between NH4+ crossover and recovery could be represented by the relative proportion (Π) between 𝐽𝑁𝐻3𝐷𝑖𝑓𝑓 and 𝐽𝑁𝐻3𝐸𝑣𝑎𝑝, which is intrinsically affected by the electrolyte pH polarization that could be manipulated by the 𝑐𝑁𝐻4+ and 𝑗. Additionally, with varied NH4+ feed locations, the anode-fed mode achieves the lowest 𝑅𝐸𝑁𝐻4+ for 𝑐𝑁𝐻4+ = 1 – 100 ppm but outperforms the cathode-fed operation for the highest 𝑐𝑁𝐻4+ (3400 ppm) due to different buffering capacities from NH4+ in the catholyte, which offers insights on selecting appropriate operation mode toward NH4+ recovery based on the 𝑐𝑁𝐻4+ in the wastewater. Furthermore, nitrogenous contaminants and membrane-bound H+ were found to interfere with the characterization of NH4+ permeability across the Nafion membrane, which could be mitigated by electrolysis conditioning in blank Na2SO4 electrolyte. Lastly, despite Celgard C3400 and C3500 158 diaphragms demonstrate better NH4+ separation at OCV than Nafion (N112/N350) or AEM (X37-50) separators, NH4+ crossover or volatilization was still found at 1-ppm level under applied 𝑗 for all the separator materials. Therefore, exploration on inert and NH4+-impermeable separators suitable for the NH4+ electrosynthesis is recommended. 159 Chapter 5 Lithium-Mediated Nitrogen Reduction in a Flow Electrolyzer Cell Using Gas Diffusion Cathode with Carbon-Based Reaction Layer 5.1 Introduction Recently, incorporation of lithium (Li) as the reaction mediator has drawn wide interests.66,70,88,110,215,252 Given limited available H+ from the organic solvent, mostly tetrahydrofuran (THF), the Li-mediated nitrogen reduction (LNRR) has shown significantly improved NH3 production rates (𝑟𝑁𝐻3> 30 µmol h−1 cm−2) and 𝐹𝐸 (> 30%) than aqueous studies.253 Isotope labelling experiments have been conducted in the LNRR system with true positive confirmations, and the accumulated 𝑐𝑁𝐻3 (sometimes mM-level) also far exceeds the contribution of background contaminants, thus allowing for prioritizing efforts in understanding and improving the system design than eliminating fictitious conclusions.31,254,255 The LNRR system commonly consists of the Li salt, non-aqueous solvent, H+ donor, and potentially a H+ shuttle. The first step in the LNRR is often referred as the electrochemical deposition of Li+ ions to metallic Li onto the cathode surface (Step 1, Equation 5.1).39,104,110,256 Then, the deposited Li as a highly active reductant chemically reacts with N2 to form lithium nitride (Step 2, Li3N, Equation 5.2).103 Lastly, Li3N as a reaction intermediate is chemically hydrolyzed by the H+ source in the electrolyte while releasing the Li+ ion (Step 3, Equation 5.3) to complete the reaction loop.104,126,257 𝐿𝑖+ + 𝑒− ↔ 𝐿𝑖 𝐸298𝐾° = −3.04 𝑉𝑆𝐻𝐸 (5.1) 3𝐿𝑖 + ½ 𝑁2 ↔ 𝐿𝑖3𝑁 𝛥𝐺298𝐾° = −135.40 𝑘𝐽 𝑚𝑜𝑙𝐿𝑖3𝑁−1 (5.2) 𝐿𝑖3𝑁 + 3𝐻+ ↔ 3𝐿𝑖+ + 𝑁𝐻3 𝛥𝐺298𝐾° = −760.98 𝑘𝐽 𝑚𝑜𝑙𝐿𝑖3𝑁−1 (5.3) 160 However, arguments exist on whether the nitridation and/or protonation steps (Step 2 and 3) are thermochemical or electrochemical in nature.104,105,109,125 For example, Schwalbe et al.109 consider the electroplated Li and its derivatives (nitride and hydride) as electrocatalysts that facilitate the direct N2 electrochemical reduction to NH3. Moreover, the kinetic model suggested by Andersen et al. 105 entails that the protonation of chemically formed Li3N undergoes the PCET, which competes with the HER and Li electroplating for the available electrons (e−) during electrolysis. Therefore, a current-cycling protocol was proposed, to plate Li during the pulsed electrolysis phase and transfer the available e− from unreacted Li to N2 during the open circuit phase, resulting in improved operational stability and 𝐹𝐸. Those two studies both provide a continuous perspective as opposed to the step-wise batch operation of the LNRR mechanism; and treat the deposited Li as a stable layer—at least transiently during the electrolysis phase—to assist the electrochemical protonation of N2 or L3N. However, the recent study by Steinberg et al.125 observed no metallic Li on the electrode in a working LNRR system using cryo-electron microscopy and suggest that Li is short-lived and cannot function as the electrocatalyst. Another possible fate of the deposited Li during the LNRR is to form the solid electrolyte interface (SEI), a passivation layer from the decomposition of electrolyte salt or solvent with Li on the cathode.215,258,259 Unlike Li ion batteries (LIB) where uniform and stable SEIs are desired to grow conformal Li underneath for the energy storage, the SEI formed in LNRR systems appears to be less structurally robust.125,258,259 The SEI, primarily altered by the addition of ethanol (EtOH, 0.5 – 2 vol%) to the THF solvent, has been assumed or characterized as porous with a moderate thickness compared to experiments with no or alternative organic additives.125,258,259 Such unique structure allows for the permeation of N2 and/or H+ source/shuttle to reach the freshly deposited Li for continuous N2 activation and protonation. 161 So far, most LNRR studies use the single-compartment batch cell (Fig. 2.14) where all the electrodes are submerged in the electrolyte. N2 in the headspace dissolves into the electrolyte as the nitrogen source at atmospheric or elevated pressure (e.g., 15 bar), whereas H+ is generally supplied from the added EtOH.252 Such design has shown advantages in simple construction, long stability and high 𝐹𝐸 (almost 100% 253), and have been used to elucidate the LNRR reaction mechanism with a static and well-controlled environment. However, as the electrodes are constantly flooded with liquid electrolyte, the 𝑟𝑁𝐻3 is often restricted by limited access to the dissolved N2 and susceptible to anodic solvent (THF or EtOH) degradation.215,252,257 To resolve those limitations, a gas-diffusion electrode (GDE) has been incorporated in a few reports using multiple-compartment cells (Fig. 2.9).39,257,260 The voids in the GDE form shortened diffusion path for the N2 mass transfer to the cathode reaction layer (RL) and plated Li, so that the LNRR performance could be elevated.222 Additionally, on the anode, it is possible to flow H2 gas to enable the hydrogen oxidation reaction (HOR, Equation 1.2) to prevent the EtOH and/or THF oxidation, while reducing the anode potential for enhanced energy efficiencies.257 Notably, the integration of the HOR on the anode also implies the advantage of conducting the LNRR continuously in a flow cell. Contrast to the batch-wise operation in three separate reactors following steps in equation 5.1 – 5.3, the continuous operation allows for the proton transfer from H2 to N2 with EtOH as the proton shuttle. Otherwise, EtOH would be consumed as the sacrificial proton source, which incurs social and economic costs to sustain the EtOH supply and reduces the attractiveness of the NH3 electrosynthesis. Despite those benefits, a flow cell electrolyzer involves complex engineering designs including the gas flow/recirculation apparatus, THF-resistant gas and liquid flow plates.39,257 Particularly, the GDE design and preparation has shown important roles in affecting the LNRR performance in 162 flow cells. So far, nearly all flow cells with GDE designs consider stainless steel cloth (SSC) as the substrate and electrocatalysts (mostly precious metals) were deposited in-situ with electro- 260 or electroless 39,257 methods. Lazouski et al.260 considered carbon paper (CP) as an unsuitable substrate compared to SSC to electrodeposit Pt as the electrocatalyst because of the susceptibility of CP to electrolyte flooding. The Pt/SSC GDEs as both the LNRR cathode and HOR anode obtain a high 𝑟𝑁𝐻3 of 108 ± 18 µmol h−1 cm−2 at −25 mA cm−2 despite a short test duration (ca. 5 min) and high cell voltages (> 20 V). The back pressure (up to 1.5 kPa) imposed at the tail gas was found essential because the resulting pressure gradient across the GDE not only effectively mitigates electrolyte crossover to the gas chamber but the electrodes are more easily exposed to N2 to facilitate the LNRR without mass transfer limitations. Fu et al.257 also deposited the PtAu alloy on SSC substrates via the hydrogen bubble templated method, which was used as both the LNRR cathode and HOR anode to afford 61 ± 1% in 𝐹𝐸 at −6 mA cm−2 continuously for up to 10 h with a current-cycling protocol. Although those flow cell applications have demonstrated highly effective and efficient NH3 electrosynthesis in non-aqueous media, the GDE preparation requires tailored deposition techniques and associates with high costs using precious metal catalysts. On the other hand, low-cost carbon-based materials have not been successfully demonstrated to fit with LNRR applications, particularly in flow cell systems. The effectiveness of carbon-based cathode toward the LNRR was only tested in a flooded cell using the CP by Gao et al.103, which obtains ca. 0.6 µmol h−1 cm−2 in 𝑟𝑁𝐻3 and 3% in 𝐹𝐸 at −8 V cell voltage for 0.5 h. Sputtering Au onto the CP was shown to improve the 𝑟𝑁𝐻3 to 2.8 ± 0.1 µmol h−1 cm−2 and ca. 10% in 𝐹𝐸 by reducing the kinetic energy barrier for the Li deposition on Au active sites. For LIB fields, lithiation on carbon-based anodes have been extensively studied in the form of intercalation and/or plating in harsh conditions 163 (e.g., charging at high 𝑗).261 Therefore, opportunities exist to incorporate carbon-based materials as lithiation/plating sites in flow cell electrolyzers toward the LNRR with carefully engineered electrode architectures and operating conditions. In this chapter, a flow cell electrolyzer containing spray-coated GDEs was used to examine the performance of different carbon-based reaction layers (carbon-supported Pt, amorphous carbon, and graphitic carbon) toward NH3 electrosynthesis in non-aqueous media. The effects of current density, EtOH concentration, and binder materials were also investigated. 5.2 Improving the LNRR performance at higher current density Initially, Pt/XC, XC, and C45 were assessed as the RL component toward the LNRR at a constant 𝑗 = −1 mA cm−2 in 1 M LiBF4 with or without EtOH. As shown in Fig. 5.1A – B, the anode potential (𝐸𝐴𝑁) at −1 mA cm−2 is relatively stable (0.1 – 0.3 VAu) regardless of the EtOH addition. The low anode overpotential for HOR is consistent with previous reports using similar flow cell configurations,257 and is significantly lower than in the case of flooded single-compartment cell under similar 𝑗 .105,106 Thus, utilizing H2 as the H+ source minimizes the decomposition of electrolyte (THF or EtOH). As the result, the electrolyte coloration and gelation, due to oxidative THF polymerization when H2 is absent from the anode gas supply (Fig. B.6 – Fig. B.8), can be largely mitigated.262 On the other hand, the cathode potential (𝐸𝐶𝐴) for Pt/XC in the RL shows large variations (> 1 V) depending on the absence or presence of EtOH. In comparison, for the carbon black cathodes (XC or C45), the 𝐸𝐶𝐴 are more stable and consistent (< 0.5 V in difference) with or without EtOH. Such observations suggest that Pt is more sensitively to the H+ concentration in the non-aqueous electrolyte, which is due to its facile HER kinetics. Hodgetts et al.263 electrochemically characterized a variety of noble metal catalysts including Pt/XC for the HOR under different H+ 164 concentrations (e.g., with and without trace H2O or 0.1 M EtOH), and observed increased cathodic 𝑗 with the EtOH addition. Therefore, it is plausible that Pt/XC also reacts with EtOH-containing electrolyte for the HER with lower overpotentials at the same 𝑗, which was observed in the current system but not for the HER-inactive XC or C45. At a current density of −1 mA cm−2, the 𝑟𝑔𝑒𝑜 and 𝐹𝐸 are extremely low (Fig. 5.1C – D) as the cathode potentials for all the investigated electrodes were below the Li deposition potential (−3.04 VRHE or −3.42 VAu). Hence, under these conditions the activation of N2 is likely to follow the heterogenous catalysis route, without the aid of electroplated highly-reductive Li. To improve the LNRR performance, the current density was increased to −5 mA cm−2 and 𝑟𝑔𝑒𝑜 improved for both 0 and 0.5 vol% EtOH cases with more enhancement for the latter. In terms of the 𝐹𝐸, only the 0.5 vol% EtOH case improves at −5 mA cm−2. The performance enhancement with Pt/XC is not obvious in comparison to the low-cost XC or C45 carbon materials, which indicates the presence of Pt might not be necessary for the LNRR. The performance enhancement with Pt/XC is not obvious in comparison to the low-cost XC or C45 carbon materials, which indicates the presence of Pt might not be necessary for the LNRR. The poor durability with the Pt/XC GDE agrees with the previous report by Hodgetts et al.263 where performance degradation was observed for Pt/XC and other Vulcan-supported precious metal catalysts (e.g., Ir, Ru). Moreover, a more active HER kinetics on those precious-metal-based catalytic sites could induce more vigorous bubble formation and hence physical disturbances detrimental to the structural integrity of the reaction layer as discussed later in Section 5.3. 165 Fig. 5.1 Electrode potentials and LNRR performances for different cathode reaction layers for the constant j operation. A – B: electrode potentials at −1 mA cm−2 with and without 0.5 vol% EtOH in the electrolyte, respectively. C – D: comparisons in rgeo and FE, respectively, for the constant j operation at −1 and −5 mA cm−2. Arrows are added as visual aids. Potentials shown are referenced to the Au pseudo-RE with manual iR-compensation. Error bars, if available, were calculated based on the standard deviation of N = 2 or 3 independent experiments. 166 Visually, after the LNRR experiments at −5 mA cm−2 without EtOH, the cathode surface that faces the electrolyte chamber shows abundant Li deposit which reacts quickly with ambient air upon the cell disassembly (Fig. 5.2A – B). However, the Li deposit notably reduces (Fig. 5.2C – E) when EtOH was present in the electrolyte. This observation agrees with the cathode appearance reported by Fu et al.257 where the presence of EtOH as the H+ shuttle facilitates the hydrolysis of Li and/or Li derivatives such as Li3N that maintains the NH3 formation. Therefore, adding EtOH to the electrolyte allows for the e− transfer toward N2 via the Li mediator instead of being fixated as under-utilized Li, which results in the enhanced 𝐹𝐸 in Fig. 5.1D.216 Nevertheless, experiments at −5 mA cm−2 result in the exfoliation of the RL for all cathode GDEs, which peel off easily from the SSC substrate after the solvent evaporation. Surprisingly, although the RL for the 0.5% EtOH test contains less Li deposit than the EtOH-absent case, the SSC Fig. 5.2 Images of the C45 GDEs (Nafion as binder) after the LNRR at −5 mA cm−2 with 0 or 0.5 vol% EtOH addition. The electrolyte-facing side is shown. A: as-prepared fresh C45 GDE. B – C: immediately and after exposing the electrode in air for 10 min, respectively, for LNRR tests with 0 vol% EtOH. D – E: immediately and after exposing the electrode in air for 10 min, respectively, for LNRR tests with 0.5 vol% EtOH. F: the reaction layer (RL) was manually removed to expose the stainless-steel cloth (SSC) substrate. 167 underneath it has significant amount of deposit (Fig. 5.2F). The detachment of cathode RL from the substrate is proposed as one of the causes for the excessive polarization observed for the high-𝑗 LNRR operation (Fig. 5.3A and Fig. 5.3C), which will be investigated further in the following section. Besides, compared to the freshly prepared electrodes (Error! Reference source not found.1), the electrolyte-facing side of the anode Pt/XC GDE acquires a surface layer while the gas-facing side of the cathode and anode largely remains intact according to the SEM imaging (Fig. E.22). The EDX analysis (Error! Reference source not found.5) shows that not only the Pt sites remains present on the anode, but the newly generated surface layer contains high percentages of boron species. It is possible that the BF4− anion in the electrolyte reacts with H+ generated by the HOR to form BF3 (𝐻+ + 𝐵𝐹4− → 𝐻𝐹 + 𝐵𝐹3) that stays on the surface.262 Such inorganic coating could incur a higher ohmic loss and contribute to the increased overpotential on the anode as the electrolysis is prolonged. 168 5.3 Investigation of Reaction Layer Exfoliation For the exfoliated region in the SEM images, a fraction of the RL remains present to cover the SSC substrate for 0 vol% EtOH in Fig. 5.3B (c), whereas the woven wires are visibly exposed in Fig. 5.3D (c) for 0.5 vol% EtOH. Besides, the high-magnification images show dendritic and spherical morphologies of the Li deposit for 0 and 0.5 vol% EtOH in Fig. 5.3B (d) and Fig. 5.3D Fig. 5.3 Performance degradation with continuous electrolysis at −5 mA cm−2 and SEM images of the C45 cathode after the exfoliation of the liquid-facing RL. A and C: electrode potentials at −5 mA cm−2 with and without 0.5% EtOH. The test duration is labelled for the respective reaction layer type. B and D: post-mortem SEM images of C45 GDEs after tests shown in A and C, respectively. Sub-figure: the exfoliated edge (a) with highlights that correspond to the exfoliated (green, b) and covered (red, c) areas; and high-magnification pictures (blue, d) of the Li deposit. 169 (d), respectively. Similar distinction was made using the cryo-SEM by Steinberg et al.125: with EtOH, the poorly passivating SEI was found to have a high porosity and poor mechanical stability possibly due to H2 formation, electrolyte swelling, and/or SEI dissolution; and contrarily, adding no EtOH results in continuous growth of Li under the dense passivating SEI. Therefore, it is hypothesized that the cathode RL exfoliation follows two possible mechanisms: (1) with no EtOH, the continuous Li deposition with negligible Li consumption by hydrolysis leads to volume expansion and fractures in the RL;264,265 (2) with EtOH, the SEI is structurally unstable due to the EtOH etching with poorly protected Li that is susceptible to mechanical disturbance such as H2 bubble formation and RL detachment from the substrate. To verify those hypotheses, half-cell experiments in a single-compartment cell (Fig. 2.14) were performed in the Ar-filled glove box using a Pt wire as the WE. Initially, the 𝐶𝑑𝑙 was measured followed by CV scans to calculate the Li plating/stripping efficiencies ( 𝜂 ). Then, the CP electrolysis at −5 mA cm−2 were performed for 0.5 h, followed by OCP for 1.0 h to observe the Li corrosion. Besides EtOH addition, the role of electrolyte (98% LiBF4) filtration was also investigated which shows negligible effects to the half-cell characterization results. In Fig. 5.4A, the 𝐶𝑑𝑙 for Pt in 0 and 0.5 vol% EtOH are relatively close, which are less than 20 µF cm−2 as reported in aqueous media 266 but agrees with the range (11 ± 5 μF cm−2) measured in acetonitrile-based non-aqueous electrolyte.267 Despite consistent 𝐶𝑑𝑙, the anodic peak for the Li stripping in Error! Reference source not found.6 drops gradually for 0.5 vol% EtOH but remains stable for 0 vol% EtOH with repeated CV scans, which corresponds to the decreasing 𝜂 for 0.5 vol% EtOH and relatively stable 𝜂 (ca. 40 – 45%) for 0 vol% EtOH, indicating more severe Li corrosion by the EtOH in the electrolyte.108 170 Furthermore, for the CP electrolysis in Fig. 5.4B, a stable and depolarizing curve was observed for 0 vol% EtOH but the 𝐸𝐶𝐴 polarizes quickly from −0.61 to −1.45 VLi+/Li during the initial 8 min for 0.5 vol% EtOH, followed by noisy 𝐸𝐶𝐴 for the remaining duration. The optical images in Fig. 5.5 and Fig. 5.6 show distinct electrode deposition and dissolution processes. For 0 vol% EtOH, the Li deposition was continuous with a thick layer clearly visible after 0.5-h electrolysis, which could expand the electrode conductive surface area with a decreasing overpotential. While the Fig. 5.4 Half-cell evaluation of the Li deposition on a Pt wire as the WE in electrolyte with and without 0.5 vol% EtOH or filtration. A: charge efficiency (η) with repeated CV cycling between −2 – −4 VAu without iR-compensation. The Cdl measured for each experiment is attached to the corresponding data set. The error bars were calculated based on the standard deviation of N ≥ 2 independent experiments. B: ECA during the CP electrolysis at −5 mA cm−2 for 0.5 h and the subsequent 1-h OCP. Note the ECA was referenced to the ELi+/Li without iR-correction. C and D: illustrations of possible cathode RL exfoliation mechanisms in electrolyte solutions with and without EtOH addition, respectively. 171 initial electrolysis (𝑡 < 8 min) also shows Li deposition for 0.5 vol% EtOH (Fig. 5.6A – E), the deposit surface appears rough and irregular possibly due to the microscopic H2 bubble formation. Considering the 𝐸𝐶𝐴 polarization in Fig. 5.4B, the Li deposit could be either loosely attached to the electrode or quickly passivated to form non-conductive SEI. However, the previously deposited Li/SEI begins to exfoliate from the electrode along with visible H2 bubble formation after 𝑡 = 10 min (Fig. 5.6F – L), until the initial Li/SEI deposit becomes largely dissolved by the end. For the following OCP, the Li chemical corrosion (and trace H2 formation) continues for 0.5 vol% EtOH along with more positive 𝐸𝐶𝐴, which differs from the stable 𝐸𝐶𝐴 close to the 𝐸𝐿𝑖+/𝐿𝑖 for 0 vol% EtOH. Fig. 5.5 Images of the lithium deposition during the chronopotentiometric electrolysis for 30 min in chronological order. Electrolyte: 1 M LiBF4 in THF with 0 vol% EtOH. j = −5 mA cm−2. 172 Hence, the volume expansion of Li deposit was confirmed in the single-compartment cell for 0 vol% EtOH, which could result in ruptures in the RL of the cathode GDE for the flow cell (Fig. 5.4C). Although Li could deposit for 0.5 vol% EtOH during the initial electrolysis, H2 evolution could occur from Li corrosion into lithium ethoxide ( 𝐿𝑖 + 𝐸𝑡𝑂𝐻 → 𝐿𝑖𝑂𝐸𝑡 +12𝐻2 125) that dissolve/disturb the mechanically instable Li/SEI deposit, leading to the RL detachment from the substrate (Fig. 5.4D). Notably, the inferior durability with 0.5 vol% (Fig. 5.3C) than without EtOH (Fig. 5.3A) also agrees with the higher overpotential for the same comparison in Fig. 5.4B. Fig. 5.6 Images of the lithium deposition during the chronopotentiometric electrolysis for 30 min in chronological order. Electrolyte: 1 M LiBF4 in THF with 0.5 vol% EtOH. j = −5 mA cm−2. 173 5.4 Optimization of the LNRR Durability The 𝑗 cycling strategy was initially proposed by Andersen et al.105 to balance the rate of Li deposition and dissolution toward N2 reduction to improve the stability of LNRR experiments in single-compartment cells, and was successfully implemented in a flow cell that operated for up to 10 h.257 Therefore, the same 𝑗 cycling strategy (1-min electrolysis followed by 1-min OCP) was applied in our flow cell system to address the RL exfoliation issue that was also linked to the Li depostion and dissolution interaction as discussed in Section 3.4. As shown in Fig. 5.7A – B, the duration of LNRR tests with and without 0.5 vol% EtOH was clearly improved by the 𝑗 cycling protocol. Interestingly, the EtOH addtion was observed to improve the number of electrolysis cycles for the cathode GDEs loaded with C45 and Pt/XC. Up to 134 cycles (equiv. 134-min electrolysis duration) were successfully completed for the C45 GDE with 0.5 vol% EtOH. Considering such improvement, a higher EtOH percentage (1.0 vol%) was tested using the same 𝑗 cycling protocol. However, the performance degradation appears to be accelerated by the excessive amount of H+ source (Fig. 5.7C) and only 65 – 71 cycles were able to be executed for the three GDEs before overloading the potentiostat. Additionally, the resting 𝐸𝐶𝐴 during OCP shows different trends depending on the presence of EtOH. As illustrated in Fig. 5.8, a relatively constant 𝐸𝐶𝐴 close to the 𝐸𝐿𝑖+/𝐿𝑖 was observed for the 0 vol% EtOH case, but adding EtOH to the electrolyte leads to rapid 𝐸𝐶𝐴 increases due to Li hydrolysis, which is 174 consistent with the comparison in the single-compartment cell between 0 and 0.5 vol% EtOH (Fig. 5.4B). In terms of the LNRR performance (Fig. 5.9 and Error! Reference source not found.7), although the improved 𝑟𝑔𝑒𝑜 and 𝐹𝐸 were observed for the 0.5 vol% EtOH than the EtOH-absent case, the 𝑗 Fig. 5.7 Electrode potentials for the LNRR tests conducted at −5 mA cm−2 with j cycling (1 min electrolysis followed by 1-min OCP resting) using different electrocatalysts and volume percentages of EtOH. A: 0 vol%; B: 0.5 vol%; C: 1.0 vol%. The total number of cycles for each test is labelled, which equals to the overall electrolysis duration in minute. 175 cycling approach does not seem to improve the activity nor the efficiency of the carbon-based GDEs. The 𝑗 cycling operation for the C45 GDE with 0.5 vol% EtOH at −5 mA cm−2 achieved 0.50 ± 0.11 μmol h−1 cm−2 and 0.81 ± 0.18% in 𝑟𝑔𝑒𝑜 and 𝐹𝐸 (𝑁 = 4), respectively, with the highest 𝑟𝑔𝑒𝑜 and 𝐹𝐸 of 0.66 μmol h−1 cm−2 and 1.06%. Further increasing the EtOH volume percentage results in a drop in both the 𝑟𝑔𝑒𝑜 and 𝐹𝐸 likely due to the excessive competing HER in the H+-rich environment, which agrees with prior studies that determined an optimial EtOH concentration less than 1 vol%.66,215 Therefore, it appears that the 𝑗 cycling strategy could improve the structural integrity of the GDEs but marginally enhance the selectivity toward the LNRR. To further improve the durability of LNRR tests in the flow cell, the 𝑗 was reduced to −3 mA cm−2 in combination with the 𝑗 cycling strategy. The C45 GDE and 0.5 vol% EtOH were applied considering the improved stability and LNRR perofrmance shown above. The reduced 𝑗 is supposed to decrease the Li deposition rate during the initial stage of electrolysis, hence reducing the volume expansion of the deposit and diminishing the H2 bubble formation. As expected, the LNRR operation was extented successfully to 8 h without significant polarization in the 𝐸𝐶𝐴 nor 𝐸𝐴𝑁 (Fig. 5.10A) and longest operation lasted for a total of 17.5 h (Error! Reference source not Fig. 5.8 Sections of the LNRR tests with j cycling at −5 mA cm−2. A: 0 vol% EtOH; B: 0.5 vol% EtOH; C: 1.0 vol% EtOH. The blue shaded area represents the OCP resting period. 176 found.8). Additionally, the increase in the ohmic resistance based on the EIS (Fig. 5.11) becomes less signifincant with −3 mA cm−2, in comparison to −5 mA cm−2. The improved physcial integrity of the RL was visually observed in Fig. 5.12C(e) with negligible cracks on the electrolyte-facing side of the C45 GDE than −5 mA cm−2 (Fig. 5.12C(i)). Furthermore, to verify NH3 production, the electrolyte and acid absorber were periodically sampled for the NH3 quantification. The NH3 yields in both the electrolyte and acid absorber both show a linear increase with the test duration at −3 mA cm−2 (Fig. 5.10B). The 𝑐𝑁𝐻3 measured in the electrolyte and in the acid absorber also continously accumulates over time (Fig. 5.10C – D), with 15.9 and 4.4 ppm NH3 obtained after the 17.5-h electrolysis, respecitvely. Notably, nearly half of the obtained NH3 was captured in the acid absorber, indicating the presence of gaseous NH3 product that was carried out by the exhasut gas, which benefits downstream separation of NH3 products from the electrolyte.39 Although the measured 𝑐𝑁𝐻3 well exceeds the common levels of background NH3 contamination (≤ 1 ppm),29 control experiments were conducted at OCP with N2 Fig. 5.9 Comparison for the LNRR performance at −5 mA cm−2 using GDEs spray-coated with Pt/XC, XC, and C45 reaction layers. A: rgeo for the constant and cycled j operations. B: FE for different volume percentages of EtOH in the electrolyte for the cycled j operation. For the C45 GDE with 0.5 vol% EtOH, four independent experiments were conducted for the error bars, and individual data points are labelled using star symbols. 177 sent to the cathode, or at −3 mA cm−2 with Ar sent to the cathode. With the C45 GDE and 0.5 vol% EtOH in the electrolyte, the Ar control test (Fig. 5.10A) demonstrates similar electrode potentials as the N2 case, despite the detected NH3 falls well below the N2 counterpart (Fig. 5.10C – D). Similarly, negligble NH3 was found from the OCP test (Error! Reference source not found.9), therefore confirming the genuine NH3 electrosynthesis in the current system. 5.5 Improving the LNRR Performance with Cathode Modifications As compared in Fig. 5.12D, although the LNRR durability was significantly improved by reducing the 𝑗 to −3 mA cm−2, the 𝑟𝑁𝐻3 (0.20 ± 0.01 μmol h−1 cm−2) and 𝐹𝐸 (0.55 ± 0.02%) for the C45 cathode GDE drops clearly from −5 mA cm−2. Therefore, modifications to the cathode RL Fig. 5.10 Results for the j cycling operation using the C45 GDE and 0.5 vol% EtOH in the electrolyte. A: continuous electrolysis for 8 h at −3 mA cm−2 with N2 or Ar to the cathode. Note that half of the duration (4 h) entails the electrolysis phase. B: continuously increased NH3 yield in the electrolyte and acid absorber for long-term electrolysis at −3 mA cm−2 (Error! Reference source not found.8). C – D: NH3 concentration in the electrolyte and acid absorber, respectively. Control experiments using OCP (4 h with N2 to the cathode) and Ar (8 h j cycling with Ar to the cathode) are also included as dashed lines. 178 including the carbon and binder materials were made to improve the LNRR performance and operational durability. Graphitic carbon has been commerically used as the LIB anode due to the advantage of selective Li+ intercalation from an edge plane to its highly ordered graphite lattice and hence higher lithiation capacity.268 Additionaly, graphite was shown to increase the electrode density under the same compression, which could improve the physcial integrity of the RL after hot-pressing and hence resists deformation due to Li plating and/or H2 bubble formation.211 Nevertheless, with randomly oritentated crystallites, carbon black shows benefits of high electrical conductivity and mitigating the Li plating at fast charging rates and therefore retains the charge capacity during cell cycling.211,261,268 For the binder choice, PVDF has been traditionally used for LIBs with high thermal and electrochemical stability. Despite being rarely studied in non-aqueous electrolyte, Nafion binder has been recently used to improve the LIB longevity with silicon anodes by forming an ionically conducitive path for facile Li+ mass transport,269 and to stablize the Li metal anode as a protective layer.270 Inspired by those previous findings, permutations were made by replacing the amorphous C45 carbon black with high-crystralline SFG graphite; and/or relacing the Nafion binder with PVDF. At −3 mA cm−2 (Fig. 5.12A), nearly all the modified cathode GDEs can complete the 8-h LNRR tests (240 cycles) without reaching the potentiostat limit, except for the SFG/Nafion GDE that lasted only 217 electrolysis cycles (ca. 7.2 h). The C45/PVDF GDE also shows increased polarization by the end of LNRR test compared to the C45/Nafion baseline. At higher 𝑗 (Fig. 5.12B), the C45/PVDF GDE reduces the LNRR durability to 80 cycles while the SFG GDEs with Nafion or PVDF binder only lasts for 50 cycles at −5 mA cm−2. 179 Additionally, different morphologies were observed for the modified cathode GDEs. In Fig. 5.12C, no RL crack or exfoliation was observed for the SFG/Nafion and SFG/PVDF GDEs at both −3 and −5 mA cm−2, where abundant Li/SEI was plated on the RL surfaces especially at −5 mA cm−2. The intact RL was expected for the SFG graphite due to a higher RL density from hot-pressuing during electrode preparation than C45 that could resist physicochemical deformations to the RL better. Fig. 5.11 EIS results for the C45 GDE (Nafion as binder) at OCP at the beginning and end of test. A: j cycling at −5 mA cm−2; B: j cycling at −3 mA cm−2. Electrolyte: 1 M LiBF4 with 0.5 vol% EtOH. The magnified results for the beginning of test EIS for −5 mA cm−2 are shown in the insert in A. 180 Fig. 5.12 LNRR performances for the modified cathodes: SFG graphite replaces amorphous C45 carbon black; and/or PVDF replaces Nafion as the binder. A – B: LNRR tests with cycled j = −3 and −5 mA cm−2, respectively. C: electrode surfaces facing the electrolyte chamber after LNRR tests. a – d: as-prepared electrode before LNRR tests. e – h: after LNRR tests with cycled j = −3 mA cm−2; i – l: after LNRR tests with cycled j = −5 mA cm−2. Electrolyte: 1 M LiBF4 with 0.5 vol% EtOH. N2 was supplied to the cathode side for all the tests. D: LNRR performances with modified cathodes. The inert shows the IB measurement of NH3 in the electrolyte (left) and acid absorber (right) with the C45/PVDF GDE. The error bars were calculated based on the standard deviation of N ≥ 2 independent experiments. 181 Moreover, the PVDF binder was found largely soluable in THF that leads to dissolution of the C45/PVDF RL at both −3 and −5 mA cm−2. Such dissolution could lead to the potential polarization and hence less durable LNRR operations for the C45/PVDF GDE. Nevertheless, in regions where the C45 (black) remains scatteredly attached, Li/SEI deposit also co-exsits that still functions as the mediator to facilitate the LNRR. Futhermore, the highest LNRR performance with 3.11 ± 0.41 μmol h−1 cm−2 in 𝑟𝑁𝐻3 and 5.0 ± 0.65% in 𝐹𝐸 were obtained with C45/PVDF GDEs at −5 mA cm−2 (Fig. 5.12D). Besides, the SFG/PVDF GDE shows roughened surface since the PVDF binder could partially dissolve in the electrolyte. The Li/SEI deposit was widely distributed on the SFG/PVDF electrode surface when the electrolysis was conducted at −5 mA cm−2. The accumulation of Li/SEI deposit on SFG-based GDEs could be explained by the susceptablility of graphitic carbon to Li plating at fast-charging conditions (i.e., high 𝑗 ).268 Instead, with amorphous C45 carbon black, Li+ intercalation could preferentially occur (6𝐿𝑖+ + 6𝑒− + 𝐶 → 𝐿𝑖6𝐶), which consumes e− that are not directed toward the Li deposition. Conversely, as Li plating tends to occur at high 𝑗, N2 is more likely to be activated with the deposited Li, thus explaining the general increasing trend in the 𝑟𝑁𝐻3 and 𝐹𝐸 with 𝑗 in Fig. 5.1C – D and Fig. 5.12D for the C45 and modified GDEs. However, the excessive Li depositon on SGF-based GDEs might also induce the SEI growth that leads to higher ohmic and charge-transfer losses, as previously discovered as one of the causes for the capacity fade in fast-charged LIBs with graphite anodes.271 Therefore, a moderate rate of Li deposition is likely to enable the activation of N2 for improved NH3 electrosynthesis, and prevent the formation of dense Li/SEI barriers that deteriorates the LNRR durability with rapidly increased overpotentials. 182 Regarding the much enhanced 𝑟𝑁𝐻3 and 𝐹𝐸 for the C45/PVDF GDE despite severe RL dissolution, it is possible that the porous structure of the electrode enables the N2 gas penetration toward the electrolyte-facing side where the electrodeposited Li is available (Fig. 5.13B). In contrast, when a dense RL is present such as the SFG-based GDEs (Fig. 5.13A), the mass transfer of N2 from the flow field toward the electrolyte chamber might become scarce and limits the LNRR performance significantly. Such hypotheses align with previous microkinetic models and experimental results that deem N2 mass transfer being one of the rate-limiting factors in achieving effective and efficient LNRR performances.105,272 Investigating the role of RL porosity and gas permeability on the LNRR performance, however, deserves to be conducted in a separate study. Lastly, the optimized LNRR performances toward NH3 production and operational durability are compared with previous publications that use flow cell electrolyzers or flooded cells containing carbon-based cathodes in Table 5.1. The 𝑟𝑁𝐻3 and 𝐹𝐸 from the C45/PVDF GDE at −5 mA cm−2 reaches a commendable level of performance in alignment with results in similar flow cell or electrode designs. Parituclarly, the C45/Nafion GDE demonstrates continous NH3 electrosynthesis at −3 mA cm−2 with advantageous durability compared to studies that sustain < 1-h stable LNRR Fig. 5.13 Schematic of N2 mass transfer through GDEs. A: dense, intact RL; B: porous, exfoliated RL. 183 operations. Admittedly, it is noteworthy that significantly higher NH3 production rates (45.5 ± 0.7 μmol h−1 cm−2) and efficiencies (61 ± 1%) with durable electrolysis operation (above 8 h) were reported recently,257 which could be attributed to better atmospheric regulation (N2-filled glove box throughout experiments), theory-guided electrocatalyst design that prevents NH3 anodic oxidation (PtAu alloy), RL deposition methods (electroless sputtering) that retains substrate porosity, and/or cell designs that achieved single-digit ohmic resistance. Nevertheless, this chapter contributes to pinpointing failure modes and establishing a baseline using low-cost carbon-based reaction layer, which implies economic viability and applicability to use flow cell electrolyzers toward the LNRR. Last but not least, when comparing the current status of aqueous ENRR in Table 1.1 and the non-aqeuous Li-mediated pathway in Table 5.1, the 𝑟𝑁𝐻3 and 𝐹𝐸 results have shown significant improvement for the latter, which are approaching the minimal requirement for industrial relevance as proposed by Giddey et al.18 While it appears promising to continue the pursuit of the LNRR, the complex cell design and operation, the costly electrolyte salt, and the social expense associated with the organic solvent inevitably impose both technical and sustainable difficulties. Accordingly, several research directions could be envisioned: (1) Model the gas-liquid-solid multi-phase interaction to improve the cell design and operating conditions with enhanced N2 mass transport and activation kinetics. (2) Search for alternative inexpensive Li salts with higher tolerance to purity (< 98%), or metallic mediators such as Na or Ca that have been proposed or preliminarily verified to serve similar roles as Li.273 (3) Develop non-toxic, environmentally-benign solvents that are stable under high anodic potentials. 184 Table 5.1 Comparison of Previous LNRR Studies using Flow Cell Electrolyzers or Flooded Cells with Carbon-based Cathodes. Cathode Anode Electrolyte 𝒓𝑵𝑯𝟑 (μmol h−1 cm−2) 𝑭𝑬 (%) 𝑬𝒄𝒆𝒍𝒍 or 𝒋 Electrolysis Duration (min) Ref. C45/PVDF on SSC Pt/XC on SSC 1 M LiBF4 with 0.5 vol% EtOH 3.11 ± 0.41 5.0 ± 0.65 𝑗 = −5 mA cm−2 80 This worka C45/Nafion on SSC Pt/XC on SSC 1 M LiBF4 with 0.5 vol% EtOH 0.20 ± 0.01 0.55 ± 0.02 𝑗 = −3 mA cm−2 > 240 This worka SSC PtAu on SSC 0.2 M LiClO4 with 0.17 M EtOH 0.45 ± 0.20 1.8 ± 0.8 𝑗 = −2 mA cm−2 52 215a Carbon paper Pt foil 0.2 M LiClO4 with 1% EtOH 0.6 ± 0.01 3 ± 0.1 𝐸𝑐𝑒𝑙𝑙 = −8 V 30 103a, b Pre-deposited Li on SSC Pt/XC on carbon paper Li+-incorprated polyethylene oxide 2.4 ± 0.3c 8.2 ± 3.7 𝐸𝑐𝑒𝑙𝑙 = −3.9 V 240d 260e PtAu on SSC PtAu on SSC 1 M LiBF4 with 0.25 vol% EtOH 45.5 ± 0.7 61 ± 1 𝑗 = −6 mA cm−2 > 240 257 Pt on SSC Pt on SSC 1 M LiBF4 with 0.11 M EtOH 108 ± 18 35 ± 6 𝑗 = −25 mA cm−2 < 5f 39 a LNRR tests were conducted out of the glove box. b Single-compartment cell with flooded electrodes. c The pre-deposited Li chemically produces a 𝑟𝑁𝐻3 of 0.5 ± 0.2 μmol h−1 cm−2 at OCV. d Rapid 𝑗 losses after ca. 60 min of initial electrolysis. e Flow cell with membrane electrolyte assemblies containing the Li+-incorprated solid electrolyte membrane. f Rapid voltage polarization to above −25 V. 185 5.6 Conclusion In summary, a flow cell configuration using GDEs containing carbon-based reaction layer was engineered to conduct the LNRR on the cathode and the HOR on the anode toward the NH3 electrosynthesis in non-aqueous electrolyte. Despite containing precious metal catalysts, Pt/XC demonstrates comparable results as the XC support or C45 carbon black and shows lowest stability at constant-𝑗 operations. The 𝑗 (−5 mA cm−2) and the EtOH concentration (0.5 vol%) were optimized while incorprating the 𝑗 cycling operation strategy to obtain the highest of 𝑟𝑁𝐻3 and 𝐹𝐸 using the C45 GDE (Nafion as binder) at 0.66 μmol h−1 cm−2 and 1.06%, respectively. Particular efforts were devoted to investigating and addressing the RL exfoliation issue and hence the performance degradation. Stable electrolysis for > 8 h were obtained at the intermediate 𝑗 of −3 mA cm−2 despite negligible improvement for LNRR performance compared to −5 mA cm−2. Furthermore, PVDF as an alternative binder to Nafion and SFG graphite as opposed to amorphous C45 carbon black were applied to modify the cathode GDE. Although the durability remains an issue for the modified cathode likely due to excessive Li plating and/or binder dissolution, the C45/PVDF GDE shows much improved LNRR performance with 3.11 ± 0.41 μmol h−1 cm−2 in 𝑟𝑁𝐻3 and 5.0 ± 0.65% in 𝐹𝐸 that are comparable to previous literatures. Promising future work includes investigating alternative carbon/binders combinations for physically robust RLs, operating the electrolyzer at appropriate 𝑗/𝐸𝐶𝐴 for balanced Li deposition and dissolution rates, and engineering the GDE architecture to faciliate the N2 mass transfer to Li for facile activation. 186 Chapter 6 Conclusions and Recommendations 6.1 Conclusions This research focuses on investigating the ENRR performance in aqueous or non-aqueous flow cell electrolyzers, addressing challenges such as false positives and poor separator selection in aqueous ENRR, and improving the NH3 electrosynthesis and operational stability in non-aqueous Li-containing electrolyte solutions. Importance of electrolyzer cell design and component optimization is highlighted, as opposed to the common approach of electrocatalyst design, toward effective and efficient NH3 electrosynthesis. The key findings and contributions from this dissertation include: For the aqueous ENRR • The background contamination can severely mask the genuine NH3 electrosynthesis in assessing the ENRR performance of Au-based cathode GDEs in the aqueous flow cell electrolyzer, as revealed by initial LSV scans in N2 or Ar atmosphere and OCV control experiments. Moreover, H+ and nitrogenous contaminants (NH4+ and NO3−) found in fresh membrane separators show pronounced impacts in skewing the assessment of NH4+ crossover. • The NH3/NH4+ crossover with the commonly used Nafion separator, as one of contributing factors to inaccurate ENRR performance evaluation, has received limited attention with non-conclusive results. By using the flow cell electrolyzer to address this concern, Nafion membranes were found ineffective at preventing the undesired NH3/NH4+ crossover even at the 1-ppm level, with a normalized flux as high as 247 µmol m−2 h−1 in Na2SO4 at −1 A m−2. • Severe NH4+ adsorption in Nafion was also observed, particularly when no supporting electrolyte was present. When counter ions (e.g., Na+) are present in the electrolyte, the NH4+ 187 desorption occurs, which could become an effective approach to assess the NH4+ contamination before or after the ENRR experiments in different electrolyte compositions. • The interactions between the Nafion membrane and NH4+ was found to depend on the electrolyte composition and pH changes in the catholyte and anolyte due to continuous electrolysis. The NH4+ crossover behaviors depending on the electrolyte at 1-ppm level are summarized in Fig. 6.1, which serves as a guidance for researchers in awareness of the consequences of the chosen electrolyte type to conduct the ENRR experiments. • By increasing the 𝑐𝑁𝐻4+ and 𝑗, the NH4+ crossover following mechanisms of NH4+ diffusion, NH4+ migration, and NH3 diffusion was clearly demonstrated, which bridges the gap in perception of NH4+ crossover in the fields of NH3 electrosynthesis and electrochemical removal. Particularly, the pH polarization across the separator was found critical in determining the 𝑅𝐸𝑁𝐻4+ from the electrolyte, which in turn is related to the 𝑐𝑁𝐻4+ (by providing buffering capacities) and 𝑗 (by consuming/generating H+ in the catholyte/anolyte). Fig. 6.1 Schematic of the interactions between the N112 membrane and the NH4+ transport in different electrolyte solutions. ΔcNH4+ = 1 ppm is described. A: H2SO4; B: NaOH; C: DI H2O; D: Na2SO4. 188 • Thicker CEM (Nafion N350), AEM (Sustainion X37-50), and microporous diaphragm (Celgard C3400, C3500, and C5550) were also examined as alternative separators, but none of them can completely avoid the NH4+ crossover under conditions toward the NH4+ electrosynthesis. The hydrophobic surface of C3400 and C3500 shows ability to reduce the NH4+ crossover only at OCV despite large ohmic losses from low ionic conductivities. For the non-aqueous ENRR • With practical relevance and scaleup possibilities, a flow cell electrolyzer that incorporates GDEs was realized to conduct the LNRR with low-cost carbon-based electrocatalysts. The RL of the cathode GDE was engineered with variations on the amorphous/graphitic carbon and Nafion/PVDF binders. With 0.5 vol% EtOH in the electrolyte, the conductive C45 carbon black GDE with the Nafion binder achieves 0.50 ± 0.11 μmol h−1 cm−2 𝑟𝑁𝐻3 with 0.81 ± 0.18% 𝐹𝐸 at cycled 𝑗 = −5 mA cm−2, which was further improved to 3.11 ± 0.41 μmol h−1 cm−2 and 5.0 ± 0.65%, respectively, with the modified C45/PVDF cathode. • Particular efforts were devoted to investigating and addressing the RL exfoliation issue and hence the performance degradation. Stable electrolysis for > 8 h were obtained at the intermediate 𝑗 of −3 mA cm−2 despite negligible improvement for LNRR performance compared to −5 mA cm−2. Insights on performance degrdation mechanisms, electrode engineering, and reactant mass transfer offer foundations for future improvements on flow cell electrolyzer designs toward the LNRR. • Overall, for both aqueous and non-aqueous NH3 electrosynthesis, designing and constructing a practical electrolytic system that can effectively and reliably assess the electrocatalyst material performances should be considered as a pre-requisite for consistent data reporting. Synergistic effects could also be possible when ideal physicochemical properties of the 189 electrolytic cell components and the electrocatalysts are combined for improved ENRR results. The following critical aspects are revealed herein when designing such electrolytic systems: o NH3/NH4+-inert separators that are free of cationic adsorbing and electrolyte bulk transport but capable of facilitating facile proton transport. o GDE architectures that are mechanically and (electro)chemically stable to endure harsh non-aqueous environments and high cell overpotentials; and porous to facilitate N2 mass transport while restricting electrolyte crossover under appropriately adjusted gas pressure gradients. o Rigorous pre-treatment/conditioning and testing procedures are critical in obtaining a stable and consistent ENRR performance. 6.2 Recommendations for Future Research Based on the above findings, the following recommendations are made to inspire further investigation and improvement of NH3 electrosynthesis from the cell design perspective: For the aqueous ENRR • Rigorous cleaning and conditioning protocols should be meticulously followed to remove possible nitrogenous contaminants and ensure consistent and genuine ENRR assessment results. It is suggested to store the prepared GDEs and CCMs in the same electrolyte solution, preferably with high cation concentrations (e.g., Na+), to remove the contaminants before installing into the flow cell electrolyzer. In-situ MEA conditioning under electrolysis conditions is also suggested to further remove residual contaminants and achieve consistent status (e.g., identity and concentration of the counter ion species) for the ion-exchange membranes. 190 • Electrolyte recirculation for both the catholyte and anolyte is recommended to monitor the NH4+ crossover, so that the origin of false positive ENRR results could be traced and resolved. Additionally, control experiments including Ar-saturated electrolysis and OCV tests should be conducted using the same MEA as the one for the ENRR performance assessment. • The flow cell configuration that allows for direct N2 gas supply to the GDE could be integrated with novel electrocatalyst development to enhance the NH3 electrosynthesis as opposed to testing electrocatalysts only in flooded H-cells. • Novel separator materials such as composite ion-exchange membranes with appropriate thickness should be developed to reduce the NH3/NH4+ crossover in flow cell electrolyzers toward NH3 electrosynthesis. The inertness against NH4+ adsorption of the separator and fluxes of NH4+ crossover could be examined following similar apparatus and experimental design as illustrated in this work. • Although the anode design was not covered in this work, its activity, stability, and cost are important in designing an integrated ENRR electrolyzer. In aqueous cells, associating the anode with synthesis of value-added chemicals, or conversion of organic pollutants can be of interest in future cell designs. Reducing the anodic NH3 oxidation also has the potential to enhance the cathode ENRR. • Investigating strategies to hydrodynamically control the multi-phase equilibrium from the cell design or operational perspectives. For example, a gas-liquid-liquid three-compartment electrolyzer (i.e., Fig. 1.4F) could be constructed to allow for the N2 transport from the gas side through the cathode GDE to the middle catholyte chamber. Under an appropriate pressure gradient across the GDE, such flow-through configuration could replace the coverage of 191 aqueous electrolyte by gaseous N2 and potentially improve the ENRR activities with less competing HER. For the non-aqueous ENRR As a newly-developed ENRR system, the LNRR cell design has raised growing interests only recently, and many aspects of the non-aqueous flow cell electrolyzer could be optimized or altered: • SSC has been widely used as the substrate in devising the GDE which could be modified with different materials such as Ti or Ni that has appeared in water electrolyzers as the porous transport layers. • The RL composition could be improved by incorporating alternative binders that appeared in LIB fields such as PTFE or styrene-butadiene rubber to achieve better RL adherence to the substrate. While it is key to maintain physical integrity, decent N2 gas permeability should also be considered for the RL engineering with examples such as hierarchical porosity design for the RL. The carbon material could be modified based on experiences from the LIB fields such as hard carbon or hollow carbon spheres. • Operating conditions were found critical in maintaining long-term operating stability for the LNRR system, which relate closely to the Li deposition and formation rates. Therefore, the 𝑗, EtOH concentration, and percentage duty cycle for the 𝑗 cycling protocol (if applicable) should be optimized for specific flow cell systems. • The electrolyte recirculation or stirring could be incorporated for the electrolyte chamber as opposed to the static electrolyte used in this study, which could disturb the Li/SEI formation and potentially influence the LNRR performance and operational durability. 192 • As EtOH is effectively acting as the H+ shuttle in the electrolyte for the current system, the distance between the cathode and anode GDE could be reduced for faster EtOH mass transfer. This is likely to improve the H+ access to the Li deposition sites for facile Li dissolution and reduce ohmic resistance that benefits the energy efficiency of the system. • The electrolyte composition could be further optimized by (1) utilizing different H+ donor/shuttle other than EtOH at optimized concentrations, (2) increasing the Li+ concentration, and (3) replacing the cation (e.g., Na+, Ca2+) and/or anion (e.g., ClO4−, OTf−) species as opposed to LiBF4. • Exploring alternative solvent in the electrolyte other than THF. The THF solvent poses a threat to the material integrity of the electrolytic system (e.g., decomposition or swelling of o-ring seals or plastic hardware), though its decomposition products (LiF) have shown positive effects in obtaining stable SEI layers benefiting continuous LNRR operation.108 Ether-based solvents such as dimethylether (DME) have demonstrated comparable LNRR performances to the THF-based electrolytes with favorable cost and compatibility competitiveness.274 • Engineering the anode GDE to improve the selectivity toward the HOR instead of potential electrolyte oxidation that leads to the anode polarization and instability. If feasible, sacrificial anode such as Al or Mg could be used to isolate the role of HOR in affecting the LNRR performance, and study the feasibility of alternative cation deposition on the cathode to activate the N2 to metal nitrides.126 • Improving the GDE structure that includes porous topography to facilitate facile N2 transport to the vicinity of electrodeposited Li and effective binder materials to improve the adhesion and cohesion of the RL on the substrate/current collector. Styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and poly(acrylic acid) (PAA) are binder candidates that have 193 appeared in LIB applications.275 Developing novel self-healing materials as the binder could be another avenue to mitigate the performance degradation related to the mechanical deformation of the RL. • Designing and synthesising electrode materials and structures capable of accommodating electrodeposited lithium by drawing inspiration from existing literature on anode-free lithium metal batteries to prevent delamination and structural damage to the LNRR cathode. 194 Bibliography 1 Fortune Business Insights, Ammonia Market Size, Share| Industry Analysis 2021 to 2026, 2021. 2 J. Lim, C. A. Fernández, S. W. Lee and M. C. Hatzell, ACS Energy Lett., 2021, 6, 3676–3685. 3 World Population Prospects 2019, https://population.un.org/wpp/Graphs/DemographicProfiles/, (accessed 15 September 2021). 4 H. Kobayashi, A. Hayakawa, K. D. K. A. Somarathne and E. C. 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Interfacial Electrochem., 1990, 284, 195–204. 206 Appendices Appendix A Supplementary Information for Chapter 1 List of Abbreviations for Appendix A: CNT Carbon Nanotube Cys Cysteine EDA Ethylenediamine G Graphene GC-MS Gas Chromatography-mass Spectrometry HHTP Hexahydroxytriphenylene IB Indophenol Blue IC Ion Chromatography ISE Ion-selective Electrode MOF Metal-organic Framework N/A Not Available PBS Phosphate Buffer Solution PEBCD Poly(N-ethyl-benzene-1,2,4,5-tetracarboxylic diimide) rGO Reduced Graphene Oxide ZIF Zeolitic Imidazolate Framework 207 Table A.1: Summary of Previous Aqueous ENRR Studies Performed in Non-separated (NS) Cells. Catalyst Substrate Electrolyte rm (μg h–1 mgcat–1) rgeo (x 10–10 mol s–1 cmgeo–2) FE (%) E vs. jNH₃ (mA cm–2) NH3 detection method Qualitative or quantitative NMR available? Test duration (h) Ref.a RuPt@C CP 1 M KOH N/A 6.37 1.2 −0.077 RHE 0.184 Nessler None 1 1 Au Au foil 0.1 M KOH N/A 0.038 0.12 −0.5 RHE 0.001 ISE None 10 2 Ni@N-C Glassy C 0.1 M KOH 106.3 13.9 11.6 −0.3 RHE 0.402 IB, Nessler None 1 3 Defective C CC 0.1 M Na2SO4, 0.02 M H2SO4 N/A 2.59 6.9 −0.3 RHE 0.075 IB None 1 4 NiGeO4@C Ni foam 0.1 M KOH 3.06 N/A 3.57 −0.1 RHE N/A IB None 2 5 Au@N-C Membrane 0.1 M HCl N/A 5.88 10.9 −0.2 RHE 0.170 IB, NMR Qualitative 3 6 Fe3O4 Ti mesh 0.1 M Na2SO4 4.63 0.56 2.6 −0.4 RHE 0.016 IB, NMR Qualitative 3 7 Au CP 0.1 M Na2SO4 40.6 0.085 18.8 −0.3 RHE 0.002 IB, NMR Qualitative 2 8 Cu@polyimide CC 0.1 M KOH 2.48 2.03 6.6 −0.3 RHE 0.059 IB, IC, NMR Qualitative 6 9 a References in Table A.1 to Table A.5 are included at the end of Appendix A to avoid confusion with the references in the main text. 208 Table A.2: Summary of Previous Aqueous ENRR Studies Performed in Liquid-Liquid (L-L) Separated Cells. Catalyst Substrate Separator Liquid electrolyte rm (μg h–1 mgcat–1) rgeo (x 10–10 mol s–1 cmgeo–2) FE (%) E vs. jNH₃ (mA cm–2) NH3 detection method Qualitative or quantitative NMR? Test duration (h) Ref. Au CP Nafion 211 0.1 M KOH 5.49 0.27 3.9 −0.2 RHE 0.008 Nessler None 3 10 Ni Ni foam Nafion 211 0.1 M LiCl/EDA (cathode) 0.05 M H2SO4 (anode) N/A 0.36 17.2 1.8 Cell voltage 0.010 GC-MS None 1 11 Mo Mo foil Nafion 117 0.01 M H2SO4 N/A 0.31 0.24 −0.49 RHE 0.009 IB None 3 12 Au@TiO2 CP Nafion 211 0.1 M HCl 21.4 1.75 8.1 −0.2 RHE 0.051 IB None 2 13 Au@CeOx-rGO CP Nafion 117 0.1 M HCl 8.3 0.27 10.1 −0.2 RHE 0.008 IB None 2 14 BiNi CP Nafion 115 0.1 M Na2SO4 17.38 1.42 13.7 −0.6 RHE 0.041 IB None 2 15 CoP CP Nafion 117 1 M KOH 21.56 1.76 0.03 −0.4 RHE 0.051 IB None 2 16 MnO Ti mesh Nafion 117 0.1 M Na2SO4 7.92 1.11 8.0 −0.39 RHE 0.032 IB, IC None 3 17 VO2 CP Nafion 211 0.1 M Na2SO4 14.85 0.49 4.0 −0.7 RHE 0.014 IB None 2.22 18 MoS2 CP Nafion 0.1 M Na2SO4 N/A 1.29 22.2 −0.25 RHE 0.037 IB None 2 19 Rh CP Nafion 211 0.1 M KOH 23.88 1.25 0.22 −0.2 RHE 0.036 IB None 2 20 Ag CP CEM 0.1 M HCl N/A 0.46 4.8 −0.6 RHE 0.013 IB None 2 21 B-C Glassy C Nafion 117 0.25 M LiClO4 33.8 N/A 39.2 −0.7 RHE N/A IB None 3 22 BiTeI Glassy C Nafion 211 0.1 M KOH N/A 0.17 1.7 −0.5 Ag/AgCl 0.005 IB None N/A 23 Mo2C@N-C CP Nafion 211 0.1 M Na2SO4 1.20 0.16 12.3 −0.2 RHE 0.005 IB None 2 24 Au@N-C CP CEM 0.1 M HCl 3.87 0.38 12.3 −0.2 RHE 0.011 IB None 2 25 Au ITO Nafion 0.5 M LiClO4 N/A 0.61 35.9 −0.4 RHE 0.018 Nessler None 12 26 PdCu@G CP Nafion 117 0.1 M KOH 2.8 N/A 0.8 −0.2 RHE N/A IB None 2 27 VN CC Nafion 0.1 M HCl 9.55 2.48 3.6 −0.3 RHE 0.072 IB None 3 28 Au@C CC Nafion 211 0.1 M KOH 1.46 N/A 6.5 −0.25 RHE N/A IB None 2 29 WO3 CP Nafion 117 0.1 M HCl 4.2 0.69 6.8 −0.12 RHE 0.020 IB None 1 30 Mo2N Glassy C Nafion 211 0.1 M HCl 78.4 4.53 4.5 −0.3 RHE 0.131 IB None 3 31 WO3 CP Nafion 117 0.1 M HCl 17.28 0.28 7 −0.3 RHE 0.008 IB None 2 32 WP2 CC Nafion 211 0.1 M KOH N/A 1.17 7.2 −0.2 RHE 0.034 Nessler, IB, IC None 3 33 Fe/Fe3O4 Fe foil Nafion 115 0.1 M PBS 0.19 0.031 8.3 −0.3 RHE 0.001 IB None 1 34 SnO2 CC Nafion 0.1 M Na2SO4 4.00 1.47 2.2 −0.7 RHE 0.043 IB None 2 35 Mn3O4 CP Nafion 211 0.1 M Na2SO4 11.6 0.38 3 −0.8 RHE 0.011 IB None 2 36 TiO2 Ti plate Nafion 211 0.1 M Na2SO4 N/A 0.92 2.5 −0.7 RHE 0.027 IB None 3 37 TiO2@rGO CP FKB-PK-130 (CEM) 0.1 M Na2SO4 15.13 0.99 3.3 −0.9 RHE 0.029 IB None 2 38 Cr2O3 CP Nafion 0.1 M Na2SO4 25.3 0.50 6.8 −0.9 RHE 0.014 IB None 2 39 Cr2O3 CP Nafion 115 0.1 M HCl 28.13 0.46 8.6 −0.75 RHE 0.013 IB None 2 40 Bi4V2O11@CeO2 CP CEM 0.1 M HCl 23.21 7.58 10.2 −0.2 RHE 0.220 IB None 2 41 209 Table A.2: continued Catalyst Substrate Separator Liquid electrolyte rm (μg h–1 mgcat–1) rgeo (x 10–10 mol s–1 cmgeo–2) FE (%) E vs. jNH₃ (mA cm–2) NH3 detection method Qualitative or Quantitative NMR? Test duration (h) Ref. Y2O3 CP FAB-PK-130 (AEM) 0.1 M Na2SO4 14.2 1.06 2.5 −0.9 RHE 0.031 IB None 2 42 Au@C CC N/A 1 M KOH 8.07 2.11 0.2 −0.5 RHE 0.061 IB None 2 43 MoS2 CC Nafion 211 0.1 M Na2SO4 N/A 0.81 1.17 −0.5 RHE 0.023 IB None 2 44 C3N4 CP FAAM-15 (AEM) 0.1 M HCl 8.09 2.64 11.6 −0.2 RHE 0.077 IB None 1.39 45 N-C CP Nafion 211 0.05 M H2SO4 23.8 2.33 1.42 −0.9 RHE 0.068 Nessler None 2 46 B-G CP Nafion 117 0.05 M H2SO4 54.9 1.60 10.8 −0.5 RHE 0.046 IB, Nessler None 2 47 MoP2O7@N/P-C CP Nafion 211 0.1 M KOH 18.7 N/A 9.0 −0.2 RHE N/A IB None 2 48 Fe2O3 CP Nafion 211 0.1 M Na2SO4 15.9 1.25 0.94 −0.8 RHE 0.036 IB None 2 49 K3Ti8O17 CP Nafion 211 0.1 M HCl 31.6 0.52 15 −0.5 RHE 0.015 IB None 2 50 Au CP CEM 0.1 M HCl 25.6 2.51 6.1 −0.2 RHE 0.073 IB None 2 51 MoS2 Glassy C Nafion 211 0.1 M Na2SO4 N/A 2.9 4.6 −0.4 RHE 0.084 IB None 2 52 FeOOH@G CP Nafion 211 0.1 M LiClO4 27.3 0.45 14.6 −0.4 RHE 0.013 IB, IC None 2 53 Au@Bi2Te3 CP Nafion 115 0.1 M Na2SO4 32.7 1.07 20.4 −0.4 RHE 0.031 IB None 2 54 Bi2O3@G C Nafion 117 0.1 M Na2SO4 5.68 0.69 11.2 −0.5 RHE 0.020 Nessler, ISE None 2 55 Co@N-C CP Nafion 115 0.1 M Na2SO4 19.2 1.57 21.8 −0.9 Ag/AgCl 0.045 IB None 3 56 PdRu CP Nafion 211 0.1 M KOH 37.2 3.67 1.85 −0.2 RHE 0.106 IB None 2 57 PdRu CP Nafion 211 0.1 M HCl 34.2 3.41 2.4 −0.2 RHE 0.099 IB None 2 58 Fe2O3 CC Nafion 211 0.1 M Na2SO4 13.6 1.11 7.7 −0.4 RHE 0.032 IB None 2 59 Y2O3-Zr CP Nafion 117 0.1 M Na2SO4 10.8 0.18 12.3 −0.5 RHE 0.005 IB None 2 60 PdH0.43 CP Nafion 211 0.1 M Na2SO4 17.5 N/A 18.8 −0.2 RHE N/A IB None 2 61 Au CP Nafion 117 1 M KOH 20 0.42 10.2 −0.2 RHE 0.012 IB, Nessler None 3 62 V@Fe2O3 CP Nafion 115 0.1 M HCl 68.7 1.12 5.7 −0.2 RHE 0.032 IB None 2 63 Ag@TiO2 CP Nafion 117 0.1 M HCl N/A 3.16 0.13 −0.6 RHE 0.091 IB None 2 64 MoS2@ZIF CP Nafion 117 0.1 M Na2SO4 56.7 4.63 30.9 −0.2 RHE 0.134 IB None 2 65 Fe3C@C RDE Nafion 211 0.05 M H2SO4 8.53 2.09 9.2 −0.2 RHE N/A IB None 2 66 La2O3 CP Nafion 0.1 M Na2SO4 17.0 0.22 4.8 −0.8 RHE 0.006 IB None 2 67 FeS@MoS2 CC Nafion 211 0.1 M Na2SO4 6.34 1.38 3.0 −0.5 RHE 0.040 IB None 2 68 MoS2@rGO CC CEM 0.1 M Na2SO4 8.65 1.13 27.9 −0.35 RHE 0.033 IB None 1 69 Mo3Fe3C CP Nafion 117 0.1 M Li2SO4 1.23 0.16 27 −0.05 RHE 0.005 IB None 2 70 Co3HHTP2 CP Nafion 211 0.5 M LiClO4 22.1 0.36 3.3 −0.4 RHE 0.010 IB None 2 71 MoFe@P-C FTO Nafion 211 0.1 M HCl 34.2 0.56 16.8 −0.5 RHE 0.016 IB None 1 72 AuCuB CP CEM 0.1 M Na2SO4 13.2 0.86 12.8 −0.5 RHE 0.025 IB None 2 73 Pd-Ag-S CP Nafion 211 0.1 M Na2SO4 9.73 0.64 18.4 −0.2 RHE 0.018 IB None 2 74 210 Table A.2: continued Catalyst Substrate Separator Liquid electrolyte rm (μg h–1 mgcat–1) rgeo (x 10–10 mol s–1 cmgeo–2) FE (%) E vs. jNH₃ (mA cm–2) NH3 detection method Qualitative or Quantitative NMR? Test duration (h) Ref. Fe@MoS2 Glassy C Nafion 211 0.1 M Na2SO4 20.1 0.79 15.7 −0.35 RHE 0.023 IB, NMR Qualitative 2 108 PdZn@N-C CP Nafion 117 0.1 M PBS 5.3 0.17 16.9 −0.2 RHE 0.005 IB, NMR Qualitative 2 109 B4C CP Nafion 115 0.1 M HCl 26.6 0.43 16 −0.75 RHE 0.013 IB, NMR Qualitative 2 110 Au@TiO2 CP Nafion 117 0.01 M HCl 64.6 N/A 7 −0.4 RHE N/A IB, IC, NMR Qualitative 2 111 Ru@N-C CP Nafion 117 0.1M HCl 11.0 3.59 15 −0.21 RHE 0.104 IB, NMR Qualitative 2 112 Fe2Mo6S8 Glassy C Nafion 211 0.5 M Na2SO4, 0.1 M citrate buffer 70 2.29 12.5 −0.2 RHE 0.066 IB, NMR Qualitative 2 113 Ti3C2 CP Nafion 0.1 M KOH 0.11 0.28 7.0 −0.2 RHE 0.008 Nessler, NMR Qualitative 3 114 Bi Cu foil Nafion 211 0.1 M HCl 5.3 0.69 10.3 −0.5 RHE 0.020 IB, IC, NMR Qualitative 2 115 B CP CEM 0.1 M Na2SO4 13.2 0.22 4.0 −0.8 RHE 0.006 IB, NMR Qualitative 2 116 fluorographene CP Nafion 117 0.1 M Na2SO4 9.3 0.30 4.2 −0.7 RHE 0.009 IB, IC, NMR Qualitative 2 117 TiO2 Ti mesh Nafion 211 0.1 M HCl N/A 1.24 9.2 −0.15 RHE 0.036 IB, IC, NMR Qualitative 3 118 MoS2 Ni foil Nafion 211 0.25 M LiClO4 N/A 10.3 27.7 −0.2 RHE 0.298 IB, NMR Qualitative 0.67 119 Mo/Mo2C@N-CNT CC CEM 5 mM H2SO4, 0.1 M K2SO4 8.1 1.32 5.9 −0.25 RHE 0.038 IB, IC, NMR Qualitative 2 120 VN Ti mesh Nafion 0.1 M HCl 3.6 0.84 2.3 −0.5 RHE 0.024 IB, NMR Qualitative 3 121 Au25-Cys-Mo CP Nafion 117 0.1 M HCl 34.5 N/A 26.5 −0.2 RHE 0.609 Nessler, IB, NMR Qualitative 3 122 P-C3N4 CP Nafion 0.1 M Na2SO4 28.7 N/A 22.2 −0.3 RHE N/A IB, NMR Qualitative 2 123 Ru@N-C Glassy C Nafion 211 0.05 M H2SO4 120.9 5.04 29.6 −0.2 RHE 0.146 IB, NMR Qualitative 2 124 MoO2-MoO3 CP Nafion 211 0.05 M H2SO4 60.9 1.99 23.8 −0.35 RHE 0.058 IB, NMR Qualitative 2 125 Al@Co3O4 Ni foam Nafion 117 0.1 M KOH N/A 0.65 6.3 −0.2 RHE 0.019 IB, NMR Quantitative 2 126 Pd-Ag-Au ITO Nafion 211 0.5 M LiClO4 N/A 2.25 44.1 −0.4 RHE 0.065 IB, NMR Quantitative 4 127 Mo2C CP Nafion 115 0.1 M HCl 35.2 11.5 6.8 −0.3 RHE 0.333 IB, NMR Quantitative 3 128 TiO2 Ti plate Nafion 0.1 M HCl 16.7 0.90 26 −0.5 RHE 0.026 IB, NMR Quantitative 2 129 Pd-Ag ITO Nafion 211 0.5 M LiClO4 N/A 7.45 19.6 −0.6 RHE 0.216 IB, NMR Quantitative 12 130 Fe@N-C Glassy C Nafion 211 0.1 M KOH 53.1 N/A 39.6 −0.35 RHE N/A IB, NMR Quantitative 2 131 NbSe2 Ni foam FKB-PK-130 0.1 M Na2SO4 89.5 14.6 6.8 −0.45 RHE 0.423 IB, IC, NMR Quantitative 2 132 MOF(NiFe)@C Glassy C Nafion 0.1 M NaHCO3 9.3 1.52 11.5 −0.347 RHE 0.044 Enzymatic kit, IB, NMR Quantitative 1 133 Fe3C/Fe2O3/Fe@C CC Nafion 115 6 M KOH 0.3 0.05 0.4 0.1 RHE 0.001 IB, IC, NMR Quantitative 0.17 134 Mo2C@C CC CEM 0.5 M Li2SO4 3.7 N/A 1.1 −0.3 RHE N/A Nessler, ISE, NMR Quantitative 2 135 Pd3Bi Glassy C FAB-PK-130 (AEM) 0.05 M H2SO4 59.1 2.32 21.5 −0.2 RHE 0.067 IB, NMR Quantitative 2 136 Mo@MnO2 CC N/A 0.1 M Na2SO4 36.6 1.20 7.9 −0.5 RHE 0.035 IB, NMR Quantitative 2 137 Pd@N-C CP Nafion 211 0.05 M H2SO4 69.2 5.65 24.8 −0.45 RHE 0.164 IB, NMR Quantitative 2 138 Ru@MoS2 CP FAAM-15 (AEM) 0.01 M HCl 5.6 0.91 12.2 −0.15 RHE 0.026 IB, NMR Quantitative 1 139 211 Table A.2: continued Catalyst Substrate Separator Liquid electrolyte rm (μg h–1 mgcat–1) rgeo (x 10–10 mol s–1 cmgeo–2) FE (%) E vs. jNH₃ (mA cm–2) NH3 detection method Qualitative or Quantitative NMR? Test duration (h) Ref. Au ITO Nafion 211 0.5 M LiClO4 N/A 0.65 14.8 −0.5 RHE 0.019 Nessler, NMR Quantitative 2 140 Au@MoSe2 CP Nafion 117 0.1 M Na2SO4 30.8 1.01 37.8 −0.3 RHE 0.029 IB, NMR Quantitative 2 141 VN@N/S-C CP Nafion 211 0.1 M HCl 20.5 N/A 8.6 −0.3 RHE N/A IB, NMR Quantitative 2 142 Fe@MoS2 CP Nafion 211 0.5 M K2SO4 8.6 1.13 18.8 −0.3 RHE 0.033 IB, NMR Quantitative 2 143 Mo@N-C CP Glass frit 0.1 M KOH 34 5.56 14.6 −0.3 RHE 0.161 Nessler, NMR Quantitative 1 144 Bi@C CP CEM 1.0 M K2SO4 3400 144.4 66 −0.5 RHE 4.181 Nessler, NMR Quantitative 2 145 Bi5O7I@C Ti plate Nafion 211 0.1 M Na2SO4, 2.25 mgcat mL−1 113.9 1.40 6.4 −0.4 RHE 0.040 IB, IC, NMR Quantitative 2 146 Table A.3: Summary of Previous Aqueous ENRR Studies Performed in Gas-Liquid (G-L) Separated Cells. Catalyst Substrate Separator Liquid electrolyte in the anode compartment rm (μg h–1 mgcat–1) rgeo (x 10–10 mol s–1 cmgeo–2) FE (%) E vs. jNH₃ (mA cm–2) NH3 detection method Qualitative or Quantitative NMR? Test duration (h) Ref. Ru C felt Nafion 2 M KOH N/A 0.034 0.28 −1.1 Ag/AgCl 0.001 IB, IC None 24 147 Pt@C CP Nafion 211 0.1 M Li2SO4 41.6 1.4 0.07 2 Cell voltage 0.039 ISE, Nessler None 1 148 Fe2O3@CNT CP Nafion 115 KHCO3 0.22 0.036 0.03 −2 Ag/AgCl 0.001 IB, ISE None 6 149 Fe2O3@CNT CP Nafion 115 0.5 M KOH 0.26 0.11 0.16 −2 Ag/AgCl 0.003 IB None 4 150 Fe2O3 CP FAA-3 (AEM) 0.5 M KOH 0.95 0.16 0.04 1.6 Cell voltage 0.005 IB, Nessler None 1 151 Fe2O3@CNT CP Nafion 115 0.1 M KOH 0.14 0.075 6.0 −0.9 Ag/AgCl 0.002 Nessler None 4 152 CrO0.66N0.56 Membrane Nafion 117 H2O 11.8 0.68 6.7 1.8 Cell voltage 0.020 ISE None 1 153 Fe2O3@CNT CP Nafion 115 0.5 M KOH 41.4 0.34 17 −0.5 RHE 0.010 IB, ISE None 2 154 Fe2O3@CNT CP Nafion 115 0.5 M KOH 24.1 N/A 3.4 −0.5 RHE N/A IB None 2 155 Ag@N-C CP Nafion 117 0.1 M HCl 270.9 8.9 19.5 −0.6 RHE 0.256 IB, IC, NMR Qualitative 2 156 Cu CP Exellion (AEM) 0.32 M KOH N/A 8.6 6.3 −0.5 RHE 0.249 Nessler, NMR Quantitative 1 157 212 Table A.4: Summary of Previous Aqueous ENRR Studies Performed in Gas-Gas (G-G) Separated Cells. Catalyst Substrate Separator Liquid electrolyte rm (μg h–1 mgcat–1) rgeo (x 10–10 mol s–1 cmgeo–2) FE (%) E vs. jNH₃ (mA cm–2) NH3 detection method Qualitative or Quantitative NMR? Test duration (h) Ref. Ru Membrane Nafion 417 N/A N/A 0.0049 0.0015 N/A N/A 0.0001 IB None N/A 158 SmFe0.7Cu0.1Ni0.2O3 Self-supporting Nafion 102 N/A N/A 113 90.4 2 Cell voltage 3.271 Nessler None N/A 159 Pt@C CP Nafion 211 N/A 189.7 31 2 0.2 Cell voltage 0.897 ISE None 1 160 Au@C CP FAAM-PK-75 (AEM) N/A 2.22 0.15 0.55 −0.4 RHE 0.004 Nessler None 1 161 Pt@C CP Celgard 3401 6 M KOH 9.91 0.40 0.011 0.5 Cell voltage 0.012 IB None 1.5 162 VN CP Nafion 211 N/A 40.5 3.3 6 −0.1 RHE 0.096 Nessler, elemental analysis, NMR Qualitative 2 163 Pd-Ag CP Nafion 211 N/A N/A 3.2 7.9 −0.07 RHE 0.092 IB, NMR Quantitative 6 130 213 Table A.5: Detailed Comparison of Experimental Procedures in Conducting Quantitative Isotope NMR Tests for the Aqueous ENRR. Ref. 15N2 purity (atom %) Gas cleaning traps (in the listed order) Repetition on quantitative NMR experiments Cross-examination with colorimetric NH3 measurements 15NH3 by NMR 14NH3 by NMR 14NH3 by colorimetric methods (IB or Nessler) 126 N/A Alkaline, acid N/A 6.54 x 10–11 mol s–1 cmgeo–2 6.67 x 10–11 mol s–1 cmgeo–2 6.48 x 10–11 mol s–1 cmgeo–2 127 98 Acid, H2O N/A 71.9 μM N/A 78.4 μM 128 N/A N/A N/A 94.8 μg h–1 mgcat–1 N/A 95.1 μg h–1 mgcat–1 129 98 Acid, H2O N/A 14.25 – 14.94 μM 16.2 – 17.01 μM 15.90 μM 130 98 Acid N/A 769.3 μM 706.7 μM 804.9 ± 65.3 μM 131 98 N/A N/A 51.86 μg h–1 mgcat–1 52.09 μg h–1 mgcat–1 53.12 μg h–1 mgcat–1 132 N/A Alkaline, FeSO4, acid N/A 260.9 μM N/A 264.4 μM 133 N/A Acid N/A 9.0 μg h–1 mgcat–1 N/A 9.3 μg h–1 mgcat–1 134 98 N/A N/A 124 ppb N/A 165 ppb 135 99 N/A N/A 4.2 μg h–1 mgcat–1 11.3 μg h–1 mgcat–1 136 99 N/A N/A 7.91 μg mL–1 or 59.3 μg h–1 mgcat–1 7.78 μg mL–1 or 58.3 μg h–1 mgcat–1 59.05 ± 2.27 μg h–1 mgcat–1 137 N/A Acid Triplet measurements for the NMR spectroscopy from the same electrolyte sample 0.378 μg mL–1 N/A 0.366 μg mL–1 138 N/A Alkaline, FeSO4, acid N/A 51.2 μM 50.7 ± 1.8 μM 50.9 μM 139 98 Acid N/A (Error bars provided) 28 ± 3 nmol or 5.5 ± 0.6 μM 30 ± 3 nmol 28 ± 3 nmol 140 N/A N/A N/A (Error bars provided) N/A N/A (below lower detection range of 100 μM) N/A (only the normalized rate was provided: 0.65 x 10–10 mol s–1 cmgeo–2) 141 99 Alkaline, FeSO4, acid N/A (Error bars provided) 33.2 ± 0.9 μg h–1 mgcat–1 31.0 ± 1.1 μg h–1 mgcat–1 30.8 μg h–1 mgcat–1 142 N/A Alkaline, acid N/A N/A 0.5 μg mL–1 N/A 143 N/A N/A N/A 0.124 μg mL–1 N/A 0.122 μg mL–1 144 N/A N/A N/A 45.2 μM N/A 46.3 ± 1.8 μM 145 N/A N/A N/A 26.7 μmol N/A 25.7 μmol 146 98 N/A N/A Values are claimed to be close, but the exact values are not disclosed explicitly. 157 N/A Acid, Alkaline N/A The FE calculated from the NMR-detected 15NH3 is 86% of that calculated from the detected NH3 via the Nessler's reagent test. 214 References for Appendix A 1 R. 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Stavitski, K. Attenkofer, J. G. Chen, Y. Yan and B. Xu, J. Am. Chem. Soc., 2018, 140, 13387–13391. 222 Appendix B Supplementary Information for Chapter 2 A B C D Fig. B.1 Flow configuration with images of hardware components. A: process flow diagram of the electrolytic system for the NH4+ crossover tests. B: the prismatic flow electrolyzer cell. C: the gas-liquid (G-L) separator D: the acid trap (ca. 6 mL) that collects exhaust NH3/NH4+ from the setup. A 2nd acid trap (25 mL) follows, but the detected NH4+ is always negligible (<0.05 ppm). 223 Fig. B.2 The gas pressure recorded at the outlet of the cathode and the anode, as well as the inlet of the 1st acid absorber (i.e., overall back pressure at the tee connector before the exhaust gas enters the acid absorbers). 224 Diagnostics on the Status of MEA based on the Cyclic Voltammetry Fig. B.3 shows the CV collected when a new MEA was installed (beginning of life, BOL) and after NH4+ crossover tests in certain electrolyte media (end of life, EOL). In H2SO4 and DI H2O, both the BOL and the EOL CVs show a similar Au oxidation peak starting from ca. 1.3 V until reaching the oxygen evolution peak at 1.6 V, and a sharp reduction peak at 1.15 V in the reverse scan.276 Besides, a wide redox peak at ca. 0.6 V can be associated with the quinone/hydroquinone reaction originated from the corrosion of unstable carbon in the carbon paper substrate in acidic electrolyte.277,278, The BOL and EOL CVs in NaOH, though similar, both deviate from the reported Au CVs in alkali from the literature.279 Intuitively, it is recognized that when the H+ activity in the liquid/solid electrolyte is low, the anode CCM can no longer effectively serve as an accurate RE, which makes it inappropriate to numerically equate the 𝐸𝑐𝑒𝑙𝑙 to the 𝐸𝑊𝐸 in the two-electrode configuration. The possible deviation of the RE from the reversible hydrogen electrode (RHE) suggests that using potentiostatic polarization may lead to inconsistent 𝐸𝑊𝐸 in different electrolyte solutions, and galvanostatic polarization (GP) should be the preferred choice to conduct the NH4+ crossover test under electrolysis conditions. Fig. B.3 CV scans of an Au GDE in the electrolyzer cell at a scan rate of 100 mV s−1 in Ar. A: beginning of life (BOL) without conditioning. B: end of life (EOL) after NH4+ crossover tests. C: in-situ pre-treatment in 2 M Na2SO4 for 12 h. Separator: Nafion N112. 225 Additionally, the resemblance between the BOL and EOL CVs suggests that the electrochemical properties of the MEA are not significantly affected by the addition of a low NH4+ concentration (1 ppm) in H2SO4, H2O, and NaOH. Therefore, freshly prepared MEAs (no conditioning) are used for the crossover tests in those electrolyte solutions. Lastly, although the BOL CV in Na2SO4 shows comparable peaks for the Au and quinone redox couples as in H2SO4 and H2O, those features are absent from the EOL CV. EDX spectroscopy reveals that Au remains present on the GDE after the crossover experiments in Na2SO4 (Fig. 2.2), which suggests that the loss of Au is not the primary reason for the diminished redox features in the CV. Nafion, due to its susceptibility to cation poisoning such as Na+, can result in decreased membrane ionic conductivity and limited availability of H+ in the catalyst layer during electrochemical processes.208,209 Hence, as Na+ accumulates within a freshly-prepared N112 membrane from the crossover experiment in Na2SO4, the local pH at the electrode surfaces may change and lead to variations in the CV scans. To confirm, a freshly prepared MEA was conditioned in-situ by recirculating concentrated Na2SO4 (2 M) to the electrolyzer cell for 12 h and collecting the CV hourly. As shown in Fig. B.3C, the initial CV in 2 M Na2SO4 is similar to the baseline CV collected in DI H2O using the unconditioned MEA. However, Au and quinone redox peaks as well as the oxygen evolution reaction all reduce gradually. After 6 h, the CV becomes stabilized without apparent redox features in the scanned region, which resembles the EOL CVs after the NH4+ crossover experiments in Na2SO4 (Fig. B.3B). Therefore, for stable electrochemical properties, the freshly assembled MEA was conditioned in 2 M Na2SO4 for at least 24 h and then soaked in 0.1 M Na2SO4 prior to NH4+ crossover tests. 226 Electrolyzer Prototype for the Non-aqueous LNRR and Optimization of Operating Conditions During the early stage of this project, a commercial electrolyzer apparatus (TP5E) from Tandem Technologies was adapted as the prototype for the preliminary LNRR experiments and optimization of operating conditions. As shown in Fig. B.5 and Fig. B.4, the prototype cell contains two stainless steel cathode plates and two titanium anode plates that correspond to the distribution of N2 and H2 gases, respectively. In the middle, a PEEK plate was designed as the liquid electrolyte chamber. The cell was compressed under pneumatic loading of 150 psig (ca. 10 barg). Fluorinated ethylene propylene (FEP)-encapsulated Viton and polytetrafluoroethylene (PTFE) sheets were used as the o-rings and gasket for sealing, respectively. The flow field pattern and liquid chamber geometry were adapted directly to designing the finalized bolted cell (Fig. 2.9), except for altering the pneumatic to bolted compression. While comparable electrochemical results were obtained for both cells (Fig. B.6), the bolted design allows for higher compression pressure without explosion hazards, prevents occasional electrolyte leakage due to non-uniform pressure distribution, and avoids chemical/electrochemical corrosion on the metallic flow field plates. As preliminary experiments with the prototype TP5E cell, the role of H+ donor in preventing the electrolyte degradation was first examined using 0.5 M LiBF4 in THF with or without the addition of EtOH and/or the supply of H2 to the anode side. The applied LiBF4 concentration is chosen to be intermediate compared to previous literature (0.1 – 2 M), which was shown to balance the electrochemical stability of the solid electrolyte interface (SEI) and the N2 solubility as proposed by Westhead et al.280. The carbon-supported Pt (Pt/XC) GDE was applied as both the cathode and anode considering the simplicity of a symmetrical cell design. 227 Fig. B.5 Images for the assembly of the prototype electrolyzer cell in the order of: stainless steel cathode manifold plate (A), stainless steel cathode flow field plate (B), PTFE gasket (C, E, G, and I), cathode gas-diffusion electrode (D), liquid chamber plate (F), anode gas-diffusion electrode (H), anode flow field plate (J), and anode manifold plate (K). The prototype cell was then installed to the electrolyzer setup via pneumatic compression as shown in Fig. B.4A. Fig. B.4 Images of the prototype electrolyzer adapted from a commercial electrolyzer setup (TP5E, Tandem Technologies). A: electrolyzer connected to the auxiliary components. B: exploded view of the prototype cell. The flow field pattern and liquid electrolyte chamber capacity were adopted directly to the design of the final electrolyzer shown in Fig. 2.9. 228 Initially, the LNRR was conducted at −1 mA cm−2 for 4 h with N2 and H2 supplied to the cathode and anode, respectively, with varying EtOH concentrations (0, 0.5, and 1.0 vol% in THF that is equivalent to 0, 0.085, and 0.17 M, respectively). After the experiments, the electrolyte color change was measured by the UV-Vis spectroscopy using a 7-mL glass vial through the wavelength between 200 – 900 nm. The viscosity of selected samples was evaluated by generating the flow curves using a modular compact rotational rheometer (MCR 302e, Anton Paar). Rotational rheometry was performed using concentric cylinders with 17-mm or 100-mm inner diameter for low- and high-viscosity samples, respectively. The zero-shear viscosity is reported as the average shear viscosity from 100 s−1 to 20 s−1. As shown in Fig. B.7B and Fig. B.8A, no obvious electrolyte color change was observed after the electrolysis when H2 is present on the anode side, regardless of the percentage of EtOH added to the electrolyte. However, among those cases, only the electrolyte with no EtOH addition Fig. B.6 Comparison between the prototype TP5E cell and the final bolted cell. A: electrode potentials for 0.5 M LiBF4 in THF with no EtOH. j = −1 mA cm−2. B: electrochemical impedance spectroscopy collected at the beginning of test (BOT) and end of test (EOT). While the geometric surface area (5 cm2) and electrolyte composition for the two cells are the same, the bolted cell shows reduced solution resistance both at BOT and EOT. Potentials shown are iR-included. 229 demonstrates notable increases in viscosity (Fig. B.8), which could be attributed to the oxidative polymerization of THF.39,215,252,257,281 The above observation is consistent with previous report from a similar flow cell design by Fu et al.257, where no color change was observed when H2 was Fig. B.7 Comparison of electrolyte color change due to the availability of H+ donor. A: blank electrolyte with different concentrations of EtOH. B: negligible electrolyte color after the LNRR at −1 mA cm−2 for 4 h when supplying H2 to the anode side, regardless of the EtOH contents in the electrolyte. C: significant electrolyte degradation as reflected by the color change after the LNRR when supplying N2 to the anode side, which also shows a decreasing color intensity with a higher EtOH percentage. D: electrolyte color change with gas switch experiment by periodically shifting between N2 (or Ar) and H2 as the gas supply to the anode. The color intensity increases gradually with time. All the above experiments were performed in the prototype TP5E cell using 0.5 M LiBF4 as the supporting electrolyte. The percentage pertains to the volume. 230 supplied to the anode side, and the addition of EtOH reduces the polymerization of THF in the electrolyte. Then, when H2 was replaced by N2 as the anode gas for the 4-h LNRR test with no EtOH addition, the electrolyte color changes into dark brown (Fig. B.7C and Fig. B.8B). Additionally, the color intensity decreases when EtOH was added to the electrolyte, which is in consistent with the correlation previously reported by Fu et al.257. The above comparisons clearly show that with no H2 nor EtOH as the H+ source, the electrolyte can be decomposed during prolong LNRR electrolysis tests, as reflected by the change in viscosity and/or color. Moreover, as compared in Fig. B.9A and Fig. B.10B, when H2 serves as the regenerative H+ source via the hydrogen oxidation reaction (HOR), both the cathode and anode potentials (𝐸𝐶𝐴 and 𝐸𝐴𝑁, Fig. B.8 Images of electrolyte color change that corresponds to the UV-Vis spectroscopy shown in Fig. B.7B – D, respectively. A: after LNRR tests with H2 sent to the anode. Left to right: 0, 0.5, and 1.0 vol% EtOH addition. B: after LNRR tests with N2 sent to the anode. Left to right: 0, 0.05, 0.5 vol% EtOH addition. C: after gas-switch experiment between N2 and H2 as the gas supply to the anode. Left: N2; right: Ar. D: gel formation due to oxidative polymerization of THF after extended ageing. Left: N2 as anode gas; right: H2 as the anode gas. E: inverted glass vials from D to show the highly viscous gel layer. 231 respectively) decrease dramatically from the case when EtOH or THF serves as sacrificial H+ donor(s). Particularly, when H2 and EtOH are both absent from the electrolyte, the anodic oxidation and subsequent polymerization of THF leads to excessive polarization in 𝐸𝐴𝑁 for 4 V within 3 h. Furthermore, gas switch experiments (Fig. B.10C) were conducted to verify the role of HOR in reducing the 𝐸𝐴𝑁 . Specifically, H2 was supplied to the anode for the initial 1-h electrolysis, followed by replacing H2 with N2 (or Ar as the control) as the anode gas for 1 h. The cycle was repeated for a total of 4 h. Interestingly, for the initial 20 – 30 min of electrolysis after switching H2 with inert gas, a mere increase of ca. 0.2 V was observed in 𝐸𝐴𝑁, which was followed by a gradual increase of ca. 0.6 V. The instantaneous 𝐸𝐴𝑁 increase upon gas switch can be explained by the positive shift in the equilibrium potential from the HOR to the THF oxidation, whereas the delay for the secondary 𝐸𝐴𝑁 increase to emerge might be related to the residual H2 dissolved in the humidifier and electrolyte during the previous hour. Eventually, when all H2 has been consumed, Fig. B.9 Viscosity (μ) measurement for the electrolyte after LNRR experiments. A: shear stress (σ) versus shear rate (γ). Results for the blank electrolyte was multiplied by 100 for visibility. B: μ calculated by linear fitting the flow curves in A. Electrolyte samples excepted for the blank are indicated in Fig. B.8. 232 the electrolyte becomes the anodic oxidation reactant and changes color after the gas-switch experiment (Fig. B.7D and Fig. B.8C). Although the color was not as intense as continuously exposing the anode to N2 throughout the 4-h electrolysis, the intensity continues to develop in the sealed glass vial post-electrochemistry (Fig. B.7D) along with an increased viscosity after days of storage (Fig. B.7D and Fig. B.8E). Therefore, from the perspective of improving the energy efficiency, maintaining electrochemical stability, and avoiding electrolyte degradation during the LNRR operation, H2 supply to the anode is required and was implemented to the operation of the bolted PEEK cell. Besides, the sensitive response in 𝐸𝐴𝑁 to the repeated gas switch operation indicates that the anode Pt/XC catalyst maintains its activity toward the HOR and is not degraded at the given 𝑗 (−1 mA cm−2). Unlike the catalyst deactivation reported previously in a flooded cell, the preservation of electrochemical activity of Pt/XC could be attributed to the imposed positive pressure gradient (15 mbar) across the GDE that expels the electrolyte from poisoning the active sites.263 Lastly, the reaction layer location and electrolyte salt concentration were studied using the electrochemical impedance spectroscopy (EIS). The reaction layer could be spray-coated onto one or both sides of the stainless-steel mesh substrate that faces the gas-, liquid- or both chambers. When measured in the prototype TP5E cell with 0.5 M LiBF4 and 0 vol% EtOH, the ohmic resistance (𝑅𝑠) for the double-sided electrodes is lower than the gas- or liquid-facing reaction layer architecture (Fig. B.11). Besides, increasing the LiBF4 concentration in THF further reduces the solution resistance to < 100 Ω. Therefore, to minimize the energy consumption and resistive heating, and allow for higher current density operation within the potentiostat load limit, electrodes with double-side reaction layers and 1 M LiBF4 were considered the default for the LNRR experiment with the bolted PEEK cell. 233 Fig. B.10 Comparison for the cathode and anode potentials (ECA and EAN, respectively) in the 4-h LNRR using the prototype TP5E cell to study the effects of H+ source to the electrolyte degradation. A: H2 was supplied to the anode. B: N2 was supplied to the anode. C: gas switch between H2 and inert gases (N2 or Ar) sent to the anode. The percentage pertains to the volume. Potentials shown are iR-included. 234 Fig. B.11 Ohmic resistance (Rs) measured by EIS at OCV using the prototype TP5E cell using different catalyst layer locations and LiBF4 concentrations. Double-sided electrodes in 1 M LiBF4 electrolyte were adopted when testing with the bolted PEEK cell. Error bars were derived from at least three repeated LNRR tests. Both the cathode and anode have the same electrode architecture for this comparison. 235 Appendix C Supplementary Information for Chapter 3 Fig. C.1 cNH3 in the electrolyte and acid absorber after the LSV and ENRR experiments with the Au/KB GDE. A: repeated LSV with gas switch between N2 and Ar for twice. B: ENRR tests at different Ecell with N2 and control experiments with OCV and Ar comparison. Fig. C.2 Repeated ENRR evaluation of the Au/KB GDE with controls. A: LSV tests with gas switch. B: ENRR electrolysis at different Ecell and control experiments with OCV and Ar comparison. NH3 in the blank electrolyte and acid solution was labelled as the reference line. 236 Appendix D Supplementary Information for Chapter 4 Fig. D.2 NH4+ desorption to blank 0.1 M Na2SO4 from a contaminated MEA membrane that was used for crossover tests in DI H2O. Fig. D.1 The crossover results from Kapton polyimide film as the separator that qualifies for no leakage in the flow cell at OCV. A: symmetric fed operation in DI H2O B: cathode-fed operation in DI H2O. 237 Fig. D.3 The percentage of NH4+ and NH3 of the total ammonia nitrogen in relation to the electrolyte pH at room temperature and pressure. Fig. D.4 Remaining NH3 that escapes from the first acid absorber but is capture by the second acid absorber after the crossover experiments with N112. A: cNH4+= 1 ppm. B: cNH4+= 3400 ppm. Note that the majority of NH3 in the exhaust gas is captured by the first acid absorber for most cases, except for the highest cNH4+ (3400 ppm) tested at the highest j (−100 A m−2). 238 Fig. D.6 NH4+ captured by the 1st acid absorber (ca. 6 mL) in the exhaust from both the cathode and anode. Comparisons are made for the N112 and N350 as the separator. The NH4+ concentration configurations (1 or 0 ppm) and the test condition (OCV or GP) are labelled in the horizontal labels. The horizontal reference lines represent the blank. Fig. D.5 Temporal pH changes for cNH4+ = 3400 ppm at −100 A m−2 for the Nafion N112 with different feed locations. 239 Fig. D.7 SEM images of as-received Celgard diaphragms with different magnifications. A – B: C3400. C – D: C5550. Note that the large voids for the microporous layer are partially covered by the non-woven fabric layer of C5550, which could result in a higher capillary pressure than C3400/C3500 for improved surface wettability. 240 Fig. D.8 Process of coat wet out for C5550. A: changes in the contact angle measurement with time from the instance when H2O was dropped to the material surface. B: time-lapse images of contact angle measurement corresponding to the time instances in A. 241 Appendix E Supplementary Information for Chapter 5 Fig. E.1 SEM images of the freshly prepared GDEs. A – B: C45 and XC (consistent morphology); C – D: Pt/XC. Note the noticeable cracks on the Pt/XC electrode after hot-pressing. 242 Fig. E.2 SEM images of the electrode surfaces after the LNRR experiments at constant j of −5 mA cm−2 A – B: gas-facing side of C45 cathode after electrolysis with 0 vol% EtOH; C – D: gas-facing side of C45 cathode after electrolysis with 0.5 vol% EtOH; E – F: electrolyte-facing side of Pt/XC anode after electrolysis with 0.5 vol% EtOH; G – H: gas-facing side of Pt/XC anode after electrolysis with 0.5 vol% EtOH. 243 Fig. E.3 Energy-dispersive X-ray spectroscopy (EDX) measurement of the electrolyte-facing side of the C45 cathode after electrolysis with 0 vol% EtOH. The region with noticeable Li deposition was selected. The energy level of the X-ray emitted from Li is too low to be included in the analysis. 244 Fig. E.4 EDX measurement of the electrolyte-facing side of the C45 cathode after electrolysis with 0 vol% EtOH. The region underneath the peel RL was selected which Li was also deposited upon. The energy level of the X-ray emitted from Li is too low to be included in the analysis. 245 Fig. E.5 EDX measurement of the electrolyte-facing side of the Pt/XC anode after electrolysis with 0 or 0.5 vol% EtOH. The region with a surface layer was selected. 246 Fig. E.6 Cyclic voltammetry (CV) scans in 1 M LiBF4 using a Pt wire as the working electrode in the single-compartment glass cell. A: without EtOH. B: with 0.5 vol% EtOH. The red arrow shows the decreasing trend of the Li stripping peak and hence the charging efficiency. The potential referes to the Au pesudo-RE and was not iR-corrected. Fig. E.7 Comparison for the LNRR performance at −5 mA cm−2 using GDEs spray-coated with Pt/XC, XC, and C45 catalysts. A: FE for the constant and pulsed j operations. B: rgeo for different volume percentages of EtOH in the electrolyte for the pulsed j operation. For the C45 GDE, three independent experiments were conducted for the error bars, and the results for the 0.5 vol% EtOH case are labelled with star symbols. 247 Fig. E.8 Long-term LNRR tests with periodic sampling for a total of 17.5 h operation duration for the C45 GDE (Nafion as binder). Note that slight performance improvement was observed when the electrolyte was sampled and refilled with fresh electrolyte. However, the electrolyte replenishment was not able to reverse the performance degradation. Fig. E.9 Control experiments at OCP with the C45 GDE (Nafion as binder) and 0.5 vol% EtOH addition for 4 h. N2 was supplied to the cathode.
Thesis/Dissertation
eng
Chemical and Biological Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Wilkinson, David P.
Gyenge, Előd
Impacts of electrolyzer design, operation, and measurement on ammonia electrosynthesis in aqueous and non-aqueous media
Text